Subzero: Improve x86-32's implementation of getGprForType(). Replaces the hacky implementation with essentially the less hacky x86-64 implementation, minus the i64 handling. Also does a couple of cleanups on the x86-64 side, including removing special-casing for rbp. BUG= none R=jpp@chromium.org Review URL: https://codereview.chromium.org/1657833002 .