KT-18731 Use reference equality to compare enums

Fix of https://youtrack.jetbrains.com/issue/KT-18731

compiler/testData/codegen/bytecodeText/enum/kt18731.kt

@@ -0,0 +1,16 @@
+enum class Bar {
+    ONE,
+    TWO
+}
+
+fun isOne(i: Bar) = i == Bar.ONE
+
+fun box(): String {
+    return when (isOne(Bar.ONE) && !isOne(Bar.TWO)) {
+        true -> "OK"
+        else -> "Failure"
+    }
+}
+
+// 1 IF_ACMPNE
+// 0 INVOKESTATIC kotlin/jvm/internal/Intrinsics.areEqual \(Ljava/lang/Object;Ljava/lang/Object;\)Z