Compile-time tuning: register/bb utilities

This CL yeilds about a 4% improvement in the compilation phase
of dex2oat (single-threaded; multi-threaded compilation is
more difficult to accurately measure).  The register utilities
could stand to be completely rewritten, but this gets most of the
easy benefit.

Next up: the assembly phase.

Change-Id: Ife5a474e9b1a6d9e501e888dda6749d34eb77e96