In a lot of cases where we were passing a subgraph (a pair of basic blocks),
we actually only needed a basic block.

Review URL: http://codereview.chromium.org/6570006

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6913 ce2b1a6d-e550-0410-aec6-3dcde31c8c00