Don't smi-untag on 64bit in the store-field stub

BUG=
R=ishell@chromium.org

Review URL: https://codereview.chromium.org/451573002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22973 ce2b1a6d-e550-0410-aec6-3dcde31c8c00