Port r12156 (e2874cdf)

Original commit message:
Allows KeyeLoad/KeyedStore operations where the key is a Smi to fold the untagging of the key into the element offset calculation.

BUG=
TEST=

Review URL: https://chromiumcodereview.appspot.com/10790143
Patch from Akos Palfi <palfia@homejinni.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12189 ce2b1a6d-e550-0410-aec6-3dcde31c8c00