Refactor transitioning stores.

There's actually no need to have the transition as part of the HStoreNamedField instruction. In fact, it is cleaner and faster to generate a separate HStoreNamedField for the transition map. This will also help to eliminate map stores with store elimination, as well as reduce register pressure for transitioning stores on ia32.

R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/295743002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21383 ce2b1a6d-e550-0410-aec6-3dcde31c8c00