Running ValueNumberingReducer together with ChangeLowering causes
trouble with the current GraphReducer and makes GVN ineffective, so
running it before we get to change lowering works around that well-known
(phase ordering) problem for now.

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/660733003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24656 ce2b1a6d-e550-0410-aec6-3dcde31c8c00