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

[via llvm-commits]

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@@ -1174,12 +1174,15 @@ void SIWholeQuadMode::toExact(MachineBasicBlock &MBB,
                               Register SaveWQM) {
   assert(LiveMaskReg.isVirtual());
 
-  bool IsTerminator = Before == MBB.end();
+  auto 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);
+    auto FirstNonDbg = skipDebugInstructionsForward(FirstTerm, MBBE);
+    if (FirstNonDbg != MBBE) {
+      auto BeforeNonDbg = skipDebugInstructionsForward(Before, MBBE);
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MarcoCastorina wrote:

Happy to change it, I was just reusing similar code from another method: https://github.com/llvm/llvm-project/pull/189370/changes?diff=split#diff-b92873bbd6b9a3411641ce2c44214e520b9c3683e4076426c3f2d8567db675c1R1109

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