Exhaustive when over enum may now use "is" instead of comparison + a pair of tests

compiler/testData/diagnostics/tests/when/ExhaustiveEnumMixed.kt

@@ -0,0 +1,11 @@
+enum class MyEnum {
+    A, B, C
+}
+
+fun foo(x: MyEnum): Int {
+    return when (x) {
+        MyEnum.A -> 1
+        is MyEnum.B -> 2
+        is MyEnum.C -> 3
+    }
+}