Optimization was only unsafe if new-space was in the low half of memory and an object could be
located in the top half at an addressed that only differ from a new-space address by the high
bit.

Review URL: http://codereview.chromium.org/159784


git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2608 ce2b1a6d-e550-0410-aec6-3dcde31c8c00