SXTW extend mode is usually cheaper on loads and stores than arithmetic,
so move it to the memory accesses where possible for Keyed loads and
stores.

BUG=
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/268483002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21172 ce2b1a6d-e550-0410-aec6-3dcde31c8c00