Made the logic architecture-independent, although we should really have some kind of instruction selection instead of trying to handle some weird cases at the hydrogen level.

Some tiny related cleanups on the way.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/141653015

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@18824 ce2b1a6d-e550-0410-aec6-3dcde31c8c00