Support subject variable in specialized code generators for 'when'

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

@@ -0,0 +1,26 @@
+// !LANGUAGE: +VariableDeclarationInWhenSubject
+// WITH_RUNTIME
+// IGNORE_BACKEND: JS
+
+fun dense(x: Int): Int {
+    return when (val xx = x) {
+        -4 -> 9
+        -1 -> 10
+        0 -> x + 11
+        1 -> 12
+        4 -> 13
+        5 -> 14
+        6 -> 15
+        7 -> 16
+        8 -> 17
+        9 -> x + 9
+        else -> 19
+    }
+}
+
+fun box(): String {
+    var result = (-5..10).map(::dense).joinToString()
+
+    if (result != "19, 9, 19, 19, 10, 11, 12, 19, 19, 13, 14, 15, 16, 17, 18, 19") return "dense:" + result
+    return "OK"
+}