[llvm] Fix assert in SlotIndexes::getInstructionIndex in debug builds (PR #189370)

[Matt Arsenault via llvm-commits]

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@@ -1174,13 +1174,17 @@ void SIWholeQuadMode::toExact(MachineBasicBlock &MBB,
                               Register SaveWQM) {
   assert(LiveMaskReg.isVirtual());
 
-  bool IsTerminator = Before == MBB.end();
+  MachineBasicBlock::iterator MBBE = MBB.end();
+  bool IsTerminator = Before == MBBE;
   if (!IsTerminator) {
     auto FirstTerm = MBB.getFirstTerminator();
-    if (FirstTerm != MBB.end()) {
-      SlotIndex FirstTermIdx = LIS->getInstructionIndex(*FirstTerm);
-      SlotIndex BeforeIdx = LIS->getInstructionIndex(*Before);
-      IsTerminator = BeforeIdx > FirstTermIdx;
+    if (FirstTerm != MBBE) {
+      MachineBasicBlock::iterator BeforeNonDbg = skipDebugInstructionsForward(Before, MBBE);
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arsenm wrote:

Shouldn't need to introduce a new level of control nesting, unconditionally use skipDebugInstructionsForward above? 

https://github.com/llvm/llvm-project/pull/189370