[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.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,