[x64] Fix optimization for certain checked load/stores.

test/mjsunit/compiler/regress-lena.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 module(stdlib, foreign, heap) {
-    "use asm";
-    var MEM32 = new stdlib.Int32Array(heap);
-    function foo(i) {
-      i = i|0;
-      MEM32[0] = i;
-      return MEM32[i + 4 >> 2]|0;
-    }
-    return { foo: foo };
-}
-
-var foo = module(this, {}, new ArrayBuffer(64*1024)).foo;
-assertEquals(-4, foo(-4));