Fix LoadFieldByIndex to take mutable heap-numbers into account.

test/mjsunit/regress/regress-load-field-by-index.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
 
-function f() {
-  var o = {x:1};
-  var y = {y:2.5, x:0};
-  var result;
-  for (var i = 0; i < 2; i++) {
-    result = o.x + 3;
-    o = y;
+var o = {a:1.5, b:{}};
+
+function f(o) {
+  var result = [];
+  for (var k in o) {
+    result[result.length] = o[k];
   }
   return result;
 }
 
-f();
-f();
+f(o);
+f(o);
 %OptimizeFunctionOnNextCall(f);
-assertEquals(3, f());
+var array = f(o);
+o.a = 1.7;
+assertEquals(1.5, array[0]);