[turbofan] Value numbering should never replace a node with itself.

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

Review URL: https://codereview.chromium.org/539833003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23724 ce2b1a6d-e550-0410-aec6-3dcde31c8c00