Fix scheduler not to connect final merge block in the graph to its inputs.

R=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/667953002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24780 ce2b1a6d-e550-0410-aec6-3dcde31c8c00