ARM64: Sign extension on MemOperand for keyed ops

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