Correctly retain argument value when deopting from Math.round on x64.

test/mjsunit/regress/regress-351624.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.
+
+// Flags: --allow-natives-syntax
+
+var big = 1e10;
+var backup = new Float64Array(1);
+
+function mult0(val){
+  var prod = val * big;
+  backup[0] = prod;
+  var rounded = Math.round(prod);
+  assertEquals(prod, backup[0]);
+  return rounded;
+}
+
+var count = 5;
+for (var i = 0; i < count; i++) {
+  if (i == count - 1) %OptimizeFunctionOnNextCall(mult0);
+  var result = mult0(-1);
+  assertEquals(result, -big);
+}