Quick compiler: Fix liveness tracking

Rework temp register liveness tracking to play nicely with aliased
physical registers, and re-enable liveness tracking optimization.

Add a pair of x86 utility routines that act like UpdateLoc(),
but only show in-register live temps if they are of the expected
register class.

Change-Id: I92779e0da2554689103e7488025be281f1a58989