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
+}

compiler/testData/codegen/box/when/whenSubjectVariable/kt27161_int.kt

@@ -0,0 +1,10 @@
+fun peek() = 1
+
+fun box(): String {
+    val x = when (val x = peek()) {
+        1 -> "OK"
+        2 -> "2"
+        else -> "other $x"
+    }
+    return x
+}

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
+}

compiler/testData/codegen/box/when/whenSubjectVariable/kt27161_nested2.kt

@@ -0,0 +1,30 @@
+enum class Test {
+    A, B, OTHER
+}
+
+fun peek() = Test.A
+
+fun box(): String {
+    val x = when (val t1 = peek()) {
+        Test.A -> {
+            when (
+                val t2 = when(val y = peek()) {
+                    Test.A -> Test.A
+                    Test.B -> Test.B
+                    else -> Test.OTHER
+                }
+            ) {
+                Test.A ->
+                    when (val t3 = peek()) {
+                        Test.A -> "OK"
+                        else -> "other 3"
+                    }
+
+                else -> "other 2"
+            }
+        }
+
+        else -> "other 1"
+    }
+    return x
+}

compiler/testData/codegen/box/when/whenSubjectVariable/kt27161_string.kt

@@ -0,0 +1,10 @@
+fun peek() = "A"
+
+fun box(): String {
+    val x = when (val s = peek()) {
+        "A" -> "OK"
+        "B" -> "B"
+        else -> "other $s"
+    }
+    return x
+}