Convert to immutable heap number when materializing arguments object.

test/mjsunit/regress/regress-457935.js

 // Copyright 2015 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 a = [];
-for (var i = -20; i < 0; ++i) {
-  a[i] = 0;
+function dummy(x) { };
+
+function g() {
+  return g.arguments;
 }
 
-function g() {
-    [].push.apply(a, arguments);
+function f(limit) {
+  var i = 0;
+  var o = {};
+  for (; i < limit; i++) {
+    o.y = +o.y;
+    g();
+  }
 }
 
-function f() {
-  g();
-}
-
-g();
-g();
+f(1);
+f(1);
 %OptimizeFunctionOnNextCall(f);
-f();
+dummy(f(1));
+dummy(f(2));