Add Schedule::InsertBranch to fuse control flow graphs.

R=jarin@chromium.org
TEST=cctest/test-schedule/TestScheduleInsertBranch

Review URL: https://codereview.chromium.org/675983002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24876 ce2b1a6d-e550-0410-aec6-3dcde31c8c00