MIPS: Fix simplification of DoLoadNamedFieldPolymorphic.  This is a commit of https://chromiumcodereview.appspot.com/10391096/ for Akos Palfi

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11572 ce2b1a6d-e550-0410-aec6-3dcde31c8c00