[AMDGPU][Scheduler] Delete RegionsWithMinOcc bitvector from scheduler (NFC) (#142361)

The `GCNScheduleDAGMILive`'s `RegionsWithMinOcc` bitvector is only used
by the `UnclusteredHighRPStage`. Its presence in the scheduler's state
forces us to maintain its value throughout scheduling even though it is
of no use to the iterative scheduling process itself. At any point
during scheduling it is possible to cheaply compute the occupancy
induced by a particular register pressure. Furthermore, the field
doesn't appear to be updated correctly throughout scheduling i.e., bits
corresponding to regions at minimum occupancy are not always set in the
vector.

This removes the bitvector from `GCNScheduleDAGMILive`.
`UnclusteredHighRPStage::initGCNRegion` now directly computes the
occupancy of possibly reschedulable regions instead of querying the
vector. Since it is the most expensive check, it is done last in the
list.