Fix using monomorphic store instruction for polymorphic stores.

R=jkummerow@chromium.org

Review URL: https://chromiumcodereview.appspot.com/16875008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15214 ce2b1a6d-e550-0410-aec6-3dcde31c8c00