Make X64 implementation update Store ic stubs.  Remove comment that talks about "below" on the stack meaning closer to the top.
Review URL: http://codereview.chromium.org/159402

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2539 ce2b1a6d-e550-0410-aec6-3dcde31c8c00