Support 'when' with subject variable in JVM BE

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

@@ -0,0 +1,10 @@
+// !LANGUAGE: +VariableDeclarationInWhenSubject
+// IGNORE_BACKEND: JS
+
+val x: Any = 1
+
+fun box() =
+    when (val y = x) {
+        is Int -> "OK"
+        else -> "Fail: $y"
+    }