Move special RPO computation into separate class.

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/673753003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24853 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Merge remote-tracking branch 'upstream/bleeding_edge' into bleeding_edge