Use the same registers for StoreIC and KeyedStoreIC.

Based on https://codereview.chromium.org/389283002 ("Use register parameters in ElementsTransitionGenerator") which must land first.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/390013002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22387 ce2b1a6d-e550-0410-aec6-3dcde31c8c00