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

test/mjsunit/asm/int16array-negative-offset.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.
+
+var stdlib = this;
+var buffer = new ArrayBuffer(64 * 1024);
+var foreign = {}
+
+
+var m = (function Module(stdlib, foreign, heap) {
+  "use asm";
+  var MEM16 = new stdlib.Int16Array(heap);
+  function load(i) {
+    i = i|0;
+    i = MEM16[i >> 1]|0;
+    return i;
+  }
+  function store(i, v) {
+    i = i|0;
+    v = v|0;
+    MEM16[i >> 1] = v;
+  }
+  function load8(i) {
+    i = i|0;
+    i = MEM16[i + 8 >> 1]|0;
+    return i;
+  }
+  function store8(i, v) {
+    i = i|0;
+    v = v|0;
+    MEM16[i + 8 >> 1] = v;
+  }
+  return { load: load, store: store, load8: load8, store8: store8 };
+})(stdlib, foreign, buffer);
+
+assertEquals(0, m.load(-8));
+assertEquals(0, m.load8(-16));
+m.store(0, 42);
+assertEquals(42, m.load8(-8));
+m.store8(-8, 99);
+assertEquals(99, m.load(0));
+assertEquals(99, m.load8(-8));