ScheduleDAGMI::initQueues changes the RegionBegin to the first non-debug instruction. Since it does not track register pressure, it does not affect any RP trackers. ScheduleDAGMILive inherits initQueues from ScheduleDAGMI, and it does reset the TopTPTracker in its schedule method. Any derived, target-specific scheduler will need to do it as well, but the TopRPTracker is only exposed as a "const" object to derived classes. Without the ability to modify the tracker directly, this leaves a derived scheduler with a potential of having the TopRPTracker out-of-sync with the CurrentTop.
The symptom of the problem:
void llvm::ScheduleDAGMILive::scheduleMI(llvm::SUnit *, bool): Assertion `TopRPTracker.getPos() == CurrentTop && "out of sync"' failed.