Fix subject variable handling in specialized enum codegen

#KT-27161

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
+}