Support 'when' with subject variable in JVM BE

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

@@ -0,0 +1,17 @@
+// !LANGUAGE: +VariableDeclarationInWhenSubject
+// IGNORE_BACKEND: JS
+
+fun box(): String {
+    var y: String = "OK"
+
+    var materializer: (() -> String)? = null
+
+    when (val x = y) {
+        "OK" -> materializer = { x }
+        else -> return "x is $x"
+    }
+
+    y = "Fail"
+
+    return materializer!!.invoke()
+}