Fix representation inference for mutable double boxes.

test/mjsunit/regress/regress-3307.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;
-  }
-  return result;
+function p(x) {
+  this.x = x;
 }
 
-f();
-f();
+function f() {
+  var a = new p(1), b = new p(2);
+  for (var i = 0; i < 1; i++) {
+    a.x += b.x;
+  }
+  return a.x;
+}
+
+new p(0.1); // make 'x' mutable box double field in p.
+
+assertEquals(3, f());
+assertEquals(3, f());
 %OptimizeFunctionOnNextCall(f);
 assertEquals(3, f());