Fix subject variable handling in specialized enum codegen

#KT-27161

compiler/testData/codegen/box/when/whenSubjectVariable/kt27161_nested.kt

@@ -0,0 +1,25 @@
+enum class Test {
+    A, B, OTHER
+}
+
+fun peek() = Test.A
+
+fun box(): String {
+    val x = when (val t1 = peek()) {
+        Test.A -> {
+            val y = peek()
+            when (val t2 = y) {
+                Test.A ->
+                    when (val t3 = peek()) {
+                        Test.A -> "OK"
+                        else -> "other 3"
+                    }
+
+                else -> "other 2"
+            }
+        }
+
+        else -> "other 1"
+    }
+    return x
+}