Specially handle the key of the LoadKeyed and StoreKeyed instruction for x32 port.

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/324913002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21826 ce2b1a6d-e550-0410-aec6-3dcde31c8c00