[turbofan] Correctify lowering of Uint8ClampedArray buffer access.

test/mjsunit/compiler/regress-444508.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";
+  // This is not valid asm.js, but should nevertheless work.
+  var MEM = new Uint8ClampedArray(heap);
+  function foo(i) { MEM[0] = 1; }
+  return {foo: foo};
+})(this, {}, new ArrayBuffer(64 * 1024)).foo();