Fix FastAssign for self-assignment

test/mjsunit/regress/regress-crbug-716520.js

Index: test/mjsunit/regress/regress-crbug-716520.js
diff --git a/test/mjsunit/regress/regress-crbug-716520.js b/test/mjsunit/regress/regress-crbug-716520.js
new file mode 100644
index 0000000000000000000000000000000000000000..5058c94a6b2239dad1ad8d3d7032c1739900651f
--- /dev/null
+++ b/test/mjsunit/regress/regress-crbug-716520.js
@@ -0,0 +1,21 @@
+// Copyright 2017 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.
+
+var __v_0 = {};
+var __v_8 = this;
+var __v_11 = -1073741825;
+__v_1 = this;
+try {
+} catch(e) {; }
+  function __f_4() {}
+  __f_4.prototype = __v_0;
+  function __f_9() { return new __f_4().v; }
+  __f_9(); __f_9();
+try {
+(function() {
+})();
+} catch(e) {; }
+  Object.assign(__v_0, __v_1, __v_0);
+(function() {
+})();