Move register conventions out of the IC classes.

A change to a convention shouldn't require recompilation of ic.h/.cc.

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/486213003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23391 ce2b1a6d-e550-0410-aec6-3dcde31c8c00