[AMDGPU] NFC: Decouple getRealRegPressure from current region (#149219)

We're already accepting a RegionIdx for the LiveIns, also use this for
the instruction iterators.

Enables querying RP for other regions -- useful for function wide
transformations (e.g. rematerialization, rewriting, etc).

Author: jrbyrnes

llvm/lib/Target/AMDGPU/GCNSchedStrategy.cpp

@@ -803,7 +803,8 @@ void GCNScheduleDAGMILive::schedule() {
 GCNRegPressure
 GCNScheduleDAGMILive::getRealRegPressure(unsigned RegionIdx) const {
   GCNDownwardRPTracker RPTracker(*LIS);
-  RPTracker.advance(begin(), end(), &LiveIns[RegionIdx]);
+  RPTracker.advance(Regions[RegionIdx].first, Regions[RegionIdx].second,
+                    &LiveIns[RegionIdx]);
   return RPTracker.moveMaxPressure();
 }