Support subject variable in specialized code generators for 'when'

compiler/testData/codegen/box/when/stringOptimization/temporaryVarInWhenByStringIsDisposedProperly.kt

@@ -0,0 +1,22 @@
+// WITH_RUNTIME
+
+val String.name get() = this
+
+fun List<String>.normalize(): List<String> {
+    val list = ArrayList<String>(this.size)
+    for (str in this) {
+        when (str.name) {
+            "." -> {}
+            ".." -> if (!list.isEmpty() && list.last().name != "..") list.removeAt(list.size - 1) else list.add(str)
+            else -> list.add(str)
+        }
+    }
+    return list
+}
+
+fun box(): String {
+    val xs = listOf("a", "b", ".", "..").normalize()
+    if (xs != listOf("a")) return "Fail: $xs"
+
+    return "OK"
+}