the frame states of all the forward-entering frames that had merge
code generated.  At a backward branch, check if there is no merge code
or if we can reuse an existing merge block.
Review URL: http://codereview.chromium.org/40169

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1502 ce2b1a6d-e550-0410-aec6-3dcde31c8c00