Port r12711 (a8d45ac2)

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

BUG=
TEST=

Review URL: https://codereview.chromium.org/11280106
Patch from Akos Palfi <palfia@homejinni.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13087 ce2b1a6d-e550-0410-aec6-3dcde31c8c00