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

[Matt Arsenault via llvm-commits]

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@@ -3982,21 +3987,31 @@ void RegisterCoalescer::lateLiveIntervalUpdate() {
 bool RegisterCoalescer::
 copyCoalesceWorkList(MutableArrayRef<MachineInstr*> CurrList) {
   bool Progress = false;
+  SmallPtrSet<MachineInstr *, 4> CurrentErasedInstrs;
   for (MachineInstr *&MI : CurrList) {
     if (!MI)
       continue;
     // Skip instruction pointers that have already been erased, for example by
     // dead code elimination.
-    if (ErasedInstrs.count(MI)) {
+    if (ErasedInstrs.count(MI) || CurrentErasedInstrs.count(MI)) {
       MI = nullptr;
       continue;
     }
     bool Again = false;
-    bool Success = joinCopy(MI, Again);
+    bool Success = joinCopy(MI, Again, CurrentErasedInstrs);
     Progress |= Success;
     if (Success || !Again)
       MI = nullptr;
   }
+  // Clear instructions not recorded in `ErasedInstrs` but erased.
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arsenm wrote:

This whole thing feels overcomplicated but I don't have any better suggestion 

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