[turbofan] Run value numbering after SimplifiedLowering.

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