should be an untagged 32-bit integer on x64. There are debug mode
asserts in the code that catch tagged integers so we get test
failures if we get it wrong.
Review URL: http://codereview.chromium.org/3928002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5668 ce2b1a6d-e550-0410-aec6-3dcde31c8c00