Fix maybe_string_add for adds that have no type feedback where --always-opt is on.

test/mjsunit/compiler/generic-add.js

+// Copyright 2015 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.
+
+function dateL() {
+  var date = new Date();
+  return (date + true) == date.toString() + true;
+}
+
+function dateR() {
+  var date = new Date();
+  return (true + date) == true + date.toString();
+}
+
+function strL() {
+  return (new String(1) + true) == "1true";
+}
+
+function strR() {
+  return (true + new String(1)) == "true1";
+}
+
+assertTrue(dateL());
+assertTrue(dateR());
+assertTrue(strL());
+assertTrue(strR());