Disable lowering to StringAdd due to various issues.

test/mjsunit/string-oom-concat.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.
 
 function concat() {
   var a = " ";
   for (var i = 0; i < 100; i++) {
     a += a;
   }
+  return a;
 }
 
 assertThrows(concat, RangeError);