This change splits the existing phi elimination into two phases:
1. Remove redundant phis
2. Remove dead phis with a fixed point iteration.

The new approach allows us to remove dead phis that are connected
in a cycle.

Review URL: http://codereview.chromium.org/6624061

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7085 ce2b1a6d-e550-0410-aec6-3dcde31c8c00