[turbofan] cleanup register allocator interface a little

BUG=
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/671043004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24978 ce2b1a6d-e550-0410-aec6-3dcde31c8c00