support for stack-allocated variables when run with multipass.

There is no liveness analysis and they are currently always allocated
to memory.

Review URL: http://codereview.chromium.org/159701

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2604 ce2b1a6d-e550-0410-aec6-3dcde31c8c00