LoadKeyedFastElement in lithium codegen.

Tested locally by hardcoding DoTaggedToI to convert smis to untagged.

Review URL: http://codereview.chromium.org/6312124

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6625 ce2b1a6d-e550-0410-aec6-3dcde31c8c00