[turbofan] Finish nodes are always marked as references.

TEST=compiler-unittests,cctest
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/533083002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23636 ce2b1a6d-e550-0410-aec6-3dcde31c8c00