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.
Reapplication of D128941/(reversion:D132037) with small fix.
why this should be setted by Stg - Stg2? is it related to the previous code?"CrossIterationNeeds.insert(std::make_pair(Reg.id(), IterNeed())).first->second |= ((1 << (OStg - Stg)) - 1);"