[llvm] [RegisterCoalescer] Deferring deletion of instructions in `ErasedInstrs` until the end of an iteration (PR #79820)

[Quentin Colombet via llvm-commits]

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@@ -2156,7 +2159,9 @@ bool RegisterCoalescer::joinCopy(MachineInstr *CopyMI, bool &Again) {
   // CopyMI has been erased by joinIntervals at this point. Remove it from
   // ErasedInstrs since copyCoalesceWorkList() won't add a successful join back
   // to the work list. This keeps ErasedInstrs from growing needlessly.
-  ErasedInstrs.erase(CopyMI);
+  if (ErasedInstrs.erase(CopyMI))
+    // But we may encounter the instruction again in this iteration.
+    CurrentErasedInstrs.insert(CopyMI);
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qcolombet wrote:

How do we end up with the same copy instruction twice in the worklist?

https://github.com/llvm/llvm-project/pull/79820