Fix subject variable handling in specialized enum codegen

#KT-27161

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
+}