Support subject variable in specialized code generators for 'when'

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

@@ -0,0 +1,19 @@
+// !LANGUAGE: +VariableDeclarationInWhenSubject
+// WITH_RUNTIME
+// IGNORE_BACKEND: JS
+
+fun sparse(x: Int): Int {
+    return when (val xx = (x % 4) * 100) {
+        100 -> 1
+        200 -> xx / 100
+        300 -> 3
+        else -> 4
+    }
+}
+
+fun box(): String {
+    var result = (0..3).map(::sparse).joinToString()
+
+    if (result != "4, 1, 2, 3") return "sparse:" + result
+    return "OK"
+}