JVM IR: Avoid unnecessary CHECKCASTs in enum classes

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

@@ -0,0 +1,12 @@
+enum class Foo {
+    A, B, C { override fun result() = "OK" };
+    open fun result() = "Fail"
+}
+
+// JVM_TEMPLATES:
+// There are two CHECKCASTs, one in Foo.valueOf and one in Foo.values
+// 2 CHECKCAST
+
+// JVM_IR_TEMPLATES:
+// There should be only one CHECKCAST in Foo.valueOf
+// 1 CHECKCAST