This makes the logic in the Hydrogen->Lithium translation much clearer, avoids a
hand-written dispatch and even opened up opportunities for simpler register
constraints for some operations/platforms.

Doing the same for the Hydrogen level might be done in a follow-up CL.

Review URL: https://codereview.chromium.org/13841003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14233 ce2b1a6d-e550-0410-aec6-3dcde31c8c00