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

[Jay Foad 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);
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jayfoad wrote:

In practice I don't think you need this part since `getFirstTerminator` should not return a debug instruction.

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