Preserve Smi representation of non-escaping fields.

R=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/251493004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20971 ce2b1a6d-e550-0410-aec6-3dcde31c8c00