This is required to enable a value numbering reducer, which needs to be
able to recognize "dead nodes".

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

Review URL: https://codereview.chromium.org/541643002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23675 ce2b1a6d-e550-0410-aec6-3dcde31c8c00