Allocate heap entry untagged and tag at end to avoid having to subtract off
the tag offset before storing the value.

BUG=

Review URL: https://codereview.chromium.org/11028115
Patch from Anthony Berent <aberent@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12711 ce2b1a6d-e550-0410-aec6-3dcde31c8c00