Verifier must ensure that start and end node of graph are set.

BUG=
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/444583004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22862 ce2b1a6d-e550-0410-aec6-3dcde31c8c00