Check elimination improvement: propagation of state through phis is supported, CheckMap narrowing implemented.

R=titzer@chromium.org

Review URL: https://codereview.chromium.org/141653009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19102 ce2b1a6d-e550-0410-aec6-3dcde31c8c00