Fix handling of potential string additions in hydrogen.

test/mjsunit/regress/regress-3476.js

+// Copyright 2014 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: --allow-natives-syntax
+
+function MyWrapper(v) {
+  return { valueOf: function() { return v } };
+}
+
+function f() {
+  assertEquals("truex", true + "x");
+  assertEquals("truey", true + new String("y"));
+  assertEquals("truez", true + new MyWrapper("z"));
+
+  assertEquals("xtrue", "x" + true);
+  assertEquals("ytrue", new String("y") + true);
+  assertEquals("ztrue", new MyWrapper("z") + true);
+}
+
+f();
+f();
+%OptimizeFunctionOnNextCall(f);
+f();