allocate and spill registers, so that the register reference counts
and virtual frame are unchanged in the main body.

This eliminates a few sites of magic branching or binding of
JumpTarget with arguments.

Review URL: http://codereview.chromium.org/119302

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2126 ce2b1a6d-e550-0410-aec6-3dcde31c8c00