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 .
3 files changed