We don't need to store the full representation for each value. Instead
a bit to indicate tagged/untagged is enough.
Review URL: http://codereview.chromium.org/9104042

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10572 ce2b1a6d-e550-0410-aec6-3dcde31c8c00