[turbofan] Fix missing ChangeUint32ToUint64 in lowering of LoadBuffer.

test/mjsunit/compiler/regress-444695.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 m = (function(stdlib, foreign, heap) {
+var foo = (function(stdlib, foreign, heap) {
   "use asm";
   var MEM = new stdlib.Uint8Array(heap);
-  function f(x) {
-    x = x | 0;
-    MEM[x] = 0;
-  }
-  return {f: f};
-})(this, {}, new ArrayBuffer(1));
-m.f(-926416896 * 32 * 1024);
+  function foo(x) { MEM[x | 0] *= 0; }
+  return {foo: foo};
+})(this, {}, new ArrayBuffer(1)).foo;
+foo(-926416896 * 8 * 1024);