[AMDGPU] Fixed crash in getLastMIForRegion when the region is empty. (#168653)

PreRARematStage builds region live-outs if GCN trackers are enabled. If
rematerialization leads to empty regions, this can cause a crash because
of dereference of an invalid iterator in getLastMIForRegion. The fix is
to skip calling getLastMIForRegion for empty regions.

This patch fixes another bug in the same code region. getLastMIForRegion
calls skipDebugInstructionsBackward which may immediately return the
RegionEnd if it is not the begin instruction and it is a non-debug
instruction. That would imply considering an instruction that is outside
the relevant region. The fix is to always pass the previous of RegionEnd
to skipDebugInstructionsBackward.

This bug was found while using GCN trackers on the existing LIT test
machine-scheduler-sink-trivial-remats.mir. Here's the assertion failure.

llvm-project/llvm/include/llvm/ADT/ilist_iterator.h:168:
llvm::ilist_iterator<OptionsT, IsReverse, IsConst>::reference
llvm::ilist_iterator<OptionsT, IsReverse, IsConst>::operator*() const
[with OptionsT = llvm::ilist_detail::node_options<llvm::MachineInstr,
true, true, void, false, void>; bool IsReverse = false; bool IsConst =
false; llvm::ilist_iterator<OptionsT, IsReverse, IsConst>::reference =
llvm::MachineInstr&]: Assertion `!NodePtr->isKnownSentinel()' failed.

llvm/lib/Target/AMDGPU/GCNSchedStrategy.cpp

@@ -978,10 +978,8 @@ GCNScheduleDAGMILive::getRealRegPressure(unsigned RegionIdx) const {
 
 static MachineInstr *getLastMIForRegion(MachineBasicBlock::iterator RegionBegin,
                                         MachineBasicBlock::iterator RegionEnd) {
-  auto REnd = RegionEnd == RegionBegin->getParent()->end()
-                  ? std::prev(RegionEnd)
-                  : RegionEnd;
-  return &*skipDebugInstructionsBackward(REnd, RegionBegin);
+  assert(RegionBegin != RegionEnd && "Region must not be empty");
+  return &*skipDebugInstructionsBackward(std::prev(RegionEnd), RegionBegin);
 }
 
 void GCNScheduleDAGMILive::computeBlockPressure(unsigned RegionIdx,
@@ -1076,9 +1074,12 @@ GCNScheduleDAGMILive::getRegionLiveOutMap() const {
   assert(!Regions.empty());
   std::vector<MachineInstr *> RegionLastMIs;
   RegionLastMIs.reserve(Regions.size());
-  for (auto &[RegionBegin, RegionEnd] : reverse(Regions))
+  for (auto &[RegionBegin, RegionEnd] : reverse(Regions)) {
+    // Skip empty regions.
+    if (RegionBegin == RegionEnd)
+      continue;
     RegionLastMIs.push_back(getLastMIForRegion(RegionBegin, RegionEnd));
-
+  }
   return getLiveRegMap(RegionLastMIs, /*After=*/true, *LIS);
 }
 
@@ -1088,10 +1089,12 @@ void RegionPressureMap::buildLiveRegMap() {
   RegionLiveRegMap =
       IsLiveOut ? DAG->getRegionLiveOutMap() : DAG->getRegionLiveInMap();
   for (unsigned I = 0; I < DAG->Regions.size(); I++) {
+    auto &[RegionBegin, RegionEnd] = DAG->Regions[I];
+    // Skip empty regions.
+    if (RegionBegin == RegionEnd)
+      continue;
     MachineInstr *RegionKey =
-        IsLiveOut
-            ? getLastMIForRegion(DAG->Regions[I].first, DAG->Regions[I].second)
-            : &*DAG->Regions[I].first;
+        IsLiveOut ? getLastMIForRegion(RegionBegin, RegionEnd) : &*RegionBegin;
     IdxToInstruction[I] = RegionKey;
   }
 }