Support subject variable in specialized code generators for 'when'

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

@@ -0,0 +1,24 @@
+// !LANGUAGE: +VariableDeclarationInWhenSubject
+// WITH_RUNTIME
+// IGNORE_BACKEND: JS
+
+import kotlin.test.assertEquals
+
+fun foo(x : String) : String {
+    return when (val xx = x) {
+        "abc", "cde" -> "1 $xx"
+        "efg", "ghi" -> "2 $xx"
+        else -> "other $xx"
+    }
+}
+
+fun box() : String {
+    assertEquals("1 abc", foo("abc"))
+    assertEquals("1 cde", foo("cde"))
+    assertEquals("2 efg", foo("efg"))
+    assertEquals("2 ghi", foo("ghi"))
+
+    assertEquals("other xyz", foo("xyz"))
+
+    return "OK"
+}