Fix subject variable handling in specialized enum codegen

#KT-27161

compiler/testData/codegen/box/when/whenSubjectVariable/kt27161.kt

@@ -0,0 +1,13 @@
+enum class Test {
+    A, B, OTHER
+}
+
+fun peek() = Test.A
+
+fun box(): String {
+    val x = when (val type = peek()) {
+        Test.A -> "OK"
+        else -> "other"
+    }
+    return x
+}