[Interpreter] Ensure ValueOf is only called once for post-increment operations.

test/mjsunit/ignition/regress-662418.js

Index: test/mjsunit/ignition/regress-662418.js
diff --git a/test/mjsunit/ignition/regress-662418.js b/test/mjsunit/ignition/regress-662418.js
new file mode 100644
index 0000000000000000000000000000000000000000..5ac3ff1952b97c56915a1c431666a64085ccb941
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+++ b/test/mjsunit/ignition/regress-662418.js
@@ -0,0 +1,18 @@
+// 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.
+
+var valueof_calls = 0;
+
+var v = {
+  toString: function() {
+    var z = w++;
+  }
+};
+var w = {
+  valueOf: function() {
+    valueof_calls++;
+  }
+};
+var x = { [v]: 'B' };
+assertTrue(valueof_calls == 1);