Scheduler checks that end block doesn't have successors.

R=jarin@chromium.org
TEST=cctest/test-scheduler/RPOLoop

Review URL: https://codereview.chromium.org/710333002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25281 ce2b1a6d-e550-0410-aec6-3dcde31c8c00