[ModuloSchedule] Add interface call to accept/reject SMS schedules

This interface allows a target to reject a proposed
SMS schedule.  For Hexagon/PowerPC, all schedules
are accepted, leaving behavior unchanged.  For ARM,
schedules which exceed register pressure limits are
rejected.

Also, two RegisterPressureTracker methods now need to be public so
that register pressure can be computed by more callers.

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D128941