[JS IR] Optimize pattern-matching of enums into comparing their ordinals

For this code:
```
enum class Season {
    WINTER,
    SPRING,
    SUMMER,
    AUTUMN
}

fun bar1(x : Season) : String {
    return when (x) {
        Season.WINTER, Season.SPRING -> "winter_spring"
        Season.SUMMER -> "summer"
        else -> "autumn"
    }
}
```

previously we generated this:
```
function foo(x) {
    var tmp0_subject = x;
    return (tmp0_subject.equals(Season_WINTER_getInstance())
        ? true
        : tmp0_subject.equals(Season_SPRING_getInstance()))
            ? 'winter_spring'
            : tmp0_subject.equals(Season_SUMMER_getInstance())
                ? 'summer'
                : 'autumn';
}
```

And now we generated this:
```
function bar2(x) {
  var tmp0_subject = x;
  var tmp0 = tmp0_subject._get_ordinal__0_k$();
  var tmp;
  switch (tmp0) {
    case 0:
    case 1:
      tmp = 'winter_spring';
      break;
    case 2:
      tmp = 'summer';
      break;
    case 3:
      tmp = 'autumn';
      break;
    default:
      noWhenBranchMatchedException();
      break;
  }
  return tmp;
}
```

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/lower/EnumWhenLowering.kt

@@ -18,10 +18,7 @@ import org.jetbrains.kotlin.ir.expressions.impl.IrGetValueImpl
 import org.jetbrains.kotlin.ir.types.classifierOrNull
 import org.jetbrains.kotlin.ir.types.getClass
 import org.jetbrains.kotlin.ir.types.isNullable
-import org.jetbrains.kotlin.ir.util.dump
+import org.jetbrains.kotlin.ir.util.*
-import org.jetbrains.kotlin.ir.util.getPropertyGetter
-import org.jetbrains.kotlin.ir.util.isNullConst
-import org.jetbrains.kotlin.ir.util.parentAsClass
 import org.jetbrains.kotlin.ir.visitors.transformChildrenVoid
 
 /** Look for when-constructs where subject is enum entry.
@@ -53,9 +50,14 @@ open class EnumWhenLowering(protected val context: CommonBackendContext) : IrEle
             return super.visitBlock(expression)
         }
         val subject = expression.statements[0]
-        if (subject !is IrVariable || subject.type.getClass()?.kind != ClassKind.ENUM_CLASS) {
+        if (subject !is IrVariable) {
             return super.visitBlock(expression)
         }
+        val subjectClass = subject.type.getClass()
+        if (subjectClass == null || subjectClass.kind != ClassKind.ENUM_CLASS || subjectClass.isEffectivelyExternal()) {
+            return super.visitBlock(expression)
+        }
+
         // Will be initialized only when we found a branch that compares
         // subject with compile-time known enum entry.
         val subjectOrdinalProvider = lazy {

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/JsLoweringPhases.kt

@@ -385,6 +385,12 @@ private val forLoopsLoweringPhase = makeBodyLoweringPhase(
     description = "[Optimization] For loops lowering"
 )
 
+private val enumWhenPhase = makeJsModulePhase(
+    ::EnumWhenLowering,
+    name = "EnumWhenLowering",
+    description = "Replace `when` subjects of enum types with their ordinals"
+).toModuleLowering()
+
 private val propertyLazyInitLoweringPhase = makeBodyLoweringPhase(
     ::PropertyLazyInitLowering,
     name = "PropertyLazyInitLowering",
@@ -832,6 +838,7 @@ private val loweringList = listOf<Lowering>(
     initializersCleanupLoweringPhase,
     kotlinNothingValueExceptionPhase,
     // Common prefix ends
+    enumWhenPhase,
     enumEntryInstancesLoweringPhase,
     enumEntryInstancesBodyLoweringPhase,
     enumClassCreateInitializerLoweringPhase,

compiler/testData/codegen/box/when/enumOptimization/bigEnum.kt

@@ -1,12 +1,8 @@
 // WITH_RUNTIME
-// CHECK_CASES_COUNT: function=bar1 count=6 TARGET_BACKENDS=JS
+// CHECK_CASES_COUNT: function=bar1 count=6
-// CHECK_CASES_COUNT: function=bar1 count=0 IGNORED_BACKENDS=JS
+// CHECK_IF_COUNT: function=bar1 count=0
-// CHECK_IF_COUNT: function=bar1 count=0 TARGET_BACKENDS=JS
+// CHECK_CASES_COUNT: function=bar2 count=6
-// CHECK_IF_COUNT: function=bar1 count=2 IGNORED_BACKENDS=JS
+// CHECK_IF_COUNT: function=bar2 count=0
-// CHECK_CASES_COUNT: function=bar2 count=6 TARGET_BACKENDS=JS
-// CHECK_CASES_COUNT: function=bar2 count=0 IGNORED_BACKENDS=JS
-// CHECK_IF_COUNT: function=bar2 count=0 TARGET_BACKENDS=JS
-// CHECK_IF_COUNT: function=bar2 count=3 IGNORED_BACKENDS=JS
 
 import kotlin.test.assertEquals
 

compiler/testData/codegen/box/when/enumOptimization/duplicatingItems.kt

@@ -1,8 +1,7 @@
 // WITH_RUNTIME
 // CHECK_CASES_COUNT: function=bar count=3 TARGET_BACKENDS=JS
-// CHECK_CASES_COUNT: function=bar count=0 IGNORED_BACKENDS=JS
+// CHECK_CASES_COUNT: function=bar count=4 IGNORED_BACKENDS=JS
-// CHECK_IF_COUNT: function=bar count=0 TARGET_BACKENDS=JS
+// CHECK_IF_COUNT: function=bar count=0
-// CHECK_IF_COUNT: function=bar count=2 IGNORED_BACKENDS=JS
 
 import kotlin.test.assertEquals
 

compiler/testData/codegen/box/when/enumOptimization/enumInsideClassObject.kt

@@ -1,6 +1,5 @@
 // WITH_RUNTIME
-// CHECK_CASES_COUNT: function=foo count=3 TARGET_BACKENDS=JS
+// CHECK_CASES_COUNT: function=foo count=3
-// CHECK_CASES_COUNT: function=foo count=0 IGNORED_BACKENDS=JS
 // CHECK_IF_COUNT: function=foo count=0
 
 import kotlin.test.assertEquals

compiler/testData/codegen/box/when/enumOptimization/expression.kt

@@ -1,11 +1,8 @@
 // WITH_RUNTIME
-// CHECK_CASES_COUNT: function=bar1 count=3 TARGET_BACKENDS=JS
+// CHECK_CASES_COUNT: function=bar1 count=3
-// CHECK_CASES_COUNT: function=bar1 count=0 IGNORED_BACKENDS=JS
 // CHECK_IF_COUNT: function=bar1 count=0
-// CHECK_CASES_COUNT: function=bar2 count=4 TARGET_BACKENDS=JS
+// CHECK_CASES_COUNT: function=bar2 count=4
-// CHECK_CASES_COUNT: function=bar2 count=0 IGNORED_BACKENDS=JS
+// CHECK_IF_COUNT: function=bar2 count=0
-// CHECK_IF_COUNT: function=bar2 count=0 TARGET_BACKENDS=JS
-// CHECK_IF_COUNT: function=bar2 count=3 IGNORED_BACKENDS=JS
 
 import kotlin.test.assertEquals
 

compiler/testData/codegen/box/when/enumOptimization/kt14597.kt

@@ -1,7 +1,5 @@
-// CHECK_CASES_COUNT: function=box count=6 TARGET_BACKENDS=JS
+// CHECK_CASES_COUNT: function=box count=6
-// CHECK_CASES_COUNT: function=box count=0 IGNORED_BACKENDS=JS
+// CHECK_IF_COUNT: function=box count=1
-// CHECK_IF_COUNT: function=box count=1 TARGET_BACKENDS=JS
-// CHECK_IF_COUNT: function=box count=7 IGNORED_BACKENDS=JS
 
 enum class En { A, B, C }
 

compiler/testData/codegen/box/when/enumOptimization/kt14597_full.kt

@@ -1,7 +1,5 @@
-// CHECK_CASES_COUNT: function=box count=18 TARGET_BACKENDS=JS
+// CHECK_CASES_COUNT: function=box count=18
-// CHECK_CASES_COUNT: function=box count=0 IGNORED_BACKENDS=JS
+// CHECK_IF_COUNT: function=box count=3
-// CHECK_IF_COUNT: function=box count=3 TARGET_BACKENDS=JS
-// CHECK_IF_COUNT: function=box count=21 IGNORED_BACKENDS=JS
 
 enum class En { A, B, C }
 

compiler/testData/codegen/box/when/enumOptimization/nullability.kt

@@ -1,8 +1,12 @@
 // WITH_RUNTIME
-// CHECK_CASES_COUNT: function=foo1 count=0
+// CHECK_CASES_COUNT: function=foo1 count=0 TARGET_BACKENDS=JS
-// CHECK_IF_COUNT: function=foo1 count=2
+// CHECK_CASES_COUNT: function=foo1 count=4 IGNORED_BACKENDS=JS
-// CHECK_CASES_COUNT: function=foo2 count=0
+// CHECK_IF_COUNT: function=foo1 count=2 TARGET_BACKENDS=JS
-// CHECK_IF_COUNT: function=foo2 count=2
+// CHECK_IF_COUNT: function=foo1 count=0 IGNORED_BACKENDS=JS
+// CHECK_CASES_COUNT: function=foo2 count=0 TARGET_BACKENDS=JS
+// CHECK_CASES_COUNT: function=foo2 count=3 IGNORED_BACKENDS=JS
+// CHECK_IF_COUNT: function=foo2 count=2 TARGET_BACKENDS=JS
+// CHECK_IF_COUNT: function=foo2 count=0 IGNORED_BACKENDS=JS
 
 import kotlin.test.assertEquals
 

compiler/testData/codegen/box/when/enumOptimization/nullableEnum.kt

@@ -1,5 +1,7 @@
-// CHECK_CASES_COUNT: function=test count=0
+// CHECK_CASES_COUNT: function=test count=0 TARGET_BACKENDS=JS
-// CHECK_IF_COUNT: function=test count=3
+// CHECK_CASES_COUNT: function=test count=3 IGNORED_BACKENDS=JS
+// CHECK_IF_COUNT: function=test count=3 TARGET_BACKENDS=JS
+// CHECK_IF_COUNT: function=test count=0 IGNORED_BACKENDS=JS
 
 enum class E {
     A,

compiler/testData/codegen/box/when/enumOptimization/withoutElse.kt

@@ -1,12 +1,10 @@
 // WITH_RUNTIME
 // CHECK_CASES_COUNT: function=bar1 count=3 TARGET_BACKENDS=JS
-// CHECK_CASES_COUNT: function=bar1 count=0 IGNORED_BACKENDS=JS
+// CHECK_CASES_COUNT: function=bar1 count=4 IGNORED_BACKENDS=JS
-// CHECK_IF_COUNT: function=bar1 count=0 TARGET_BACKENDS=JS
+// CHECK_IF_COUNT: function=bar1 count=0
-// CHECK_IF_COUNT: function=bar1 count=3 IGNORED_BACKENDS=JS
 // CHECK_CASES_COUNT: function=bar2 count=4 TARGET_BACKENDS=JS
-// CHECK_CASES_COUNT: function=bar2 count=0 IGNORED_BACKENDS=JS
+// CHECK_CASES_COUNT: function=bar2 count=5 IGNORED_BACKENDS=JS
-// CHECK_IF_COUNT: function=bar2 count=0 TARGET_BACKENDS=JS
+// CHECK_IF_COUNT: function=bar2 count=0
-// CHECK_IF_COUNT: function=bar2 count=4 IGNORED_BACKENDS=JS
 
 import kotlin.test.assertEquals
 

js/js.translator/testData/box/expression/when/externalEnumSubject.kt

@@ -1,4 +1,5 @@
 // EXPECTED_REACHABLE_NODES: 1291
+// CHECK_IF_COUNT: function=foo count=2
 external enum class X {
     A, B
 }