Improve dead code elimination by transitively marking live code and removing all dead code. Replace unreachable phi removal algorithm with the new dead code elimination pass, which is more thorough.

Review URL: https://codereview.chromium.org/14676011

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14661 ce2b1a6d-e550-0410-aec6-3dcde31c8c00