Fix bug with re-scoping arrow function parameter initializers

test/mjsunit/regress/regress-622663.js

++// Copyright 2016 the V8 project authors. All rights reserved.
++// Use of this source code is governed by a BSD-style license that can be
++// found in the LICENSE file.
++
++// Flags: --no-lazy
+
+(function() {
+  try { (y = [...[]]) => {} } catch(_) {}  // will core dump, if not fixed
+})();
+
+(function() {
+  try { ((y = [...[]]) => {})(); } catch(_) {}  // will core dump, if not fixed,
+                                                // even without --no-lazy
+})();