when: emit lookupswitch/tableswitch if possible

A lookupswitch or tableswitch can be used if all conditions are equality
checks to constants. To be more specific, it can be done if:
  1. All conditions are CALL 'EQEQ(Any?, Any?)': Boolean
  2. All types of variables involved in comparison are in the same group
     of Char/Byte/Short/Int, String or enum.
  3. All arg0 refer to the same value.
  4. All arg1 are IrConst<*>.

Change-Id: Ifd7cb618395f6c5cc64601018b446f0bb7f5891c

compiler/backend/src/org/jetbrains/kotlin/codegen/when/SwitchCodegen.kt

@@ -186,12 +186,7 @@ abstract class SwitchCodegen(
         val minValue = keys.first()
         val rangeLength = maxValue.toLong() - minValue.toLong() + 1L
 
-        // In modern JVM implementations it shouldn't matter very much for runtime performance
-        // whether to choose lookupswitch or tableswitch.
-        // The only metric that really matters is bytecode size and here we can estimate:
-        // - lookupswitch: ~ 2 * labelsNumber
-        // - tableswitch: ~ rangeLength
-        if (rangeLength > 2L * labelsNumber || rangeLength > Int.MAX_VALUE) {
+        if (preferLookupOverSwitch(labelsNumber, rangeLength)) {
             val labels = transitionsTable.values.toTypedArray()
             v.lookupswitch(defaultLabel, keys, labels)
             return
@@ -219,4 +214,13 @@ abstract class SwitchCodegen(
             }
         }
     }
+
+    companion object {
+        // In modern JVM implementations it shouldn't matter very much for runtime performance
+        // whether to choose lookupswitch or tableswitch.
+        // The only metric that really matters is bytecode size and here we can estimate:
+        // - lookupswitch: ~ 2 * labelsNumber
+        // - tableswitch: ~ rangeLength
+        fun preferLookupOverSwitch(labelsNumber: Int, rangeLength: Long) = rangeLength > 2L * labelsNumber || rangeLength > Int.MAX_VALUE
+    }
 }