Store transition on HStoreNamedField as HConstant.

This allows optimization passes that run in the parallel compiler thread
to use the map that a store transitions to for further analysis even
though the map handle cannot be dereferenced.

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/21560002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16028 ce2b1a6d-e550-0410-aec6-3dcde31c8c00