More check elimination improvements including partial learning on false branches of CompareMap and better handling of unreachable blocks.

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/159963002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19300 ce2b1a6d-e550-0410-aec6-3dcde31c8c00