[llvm] [Peephole] Check instructions from CopyMIs are still COPY (PR #69511)

[via llvm-commits]

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@@ -1445,7 +1445,9 @@ bool PeepholeOptimizer::foldRedundantCopy(
   }
 
   MachineInstr *PrevCopy = CopyMIs.find(SrcPair)->second;
-  if (!LocalMIs.count(PrevCopy))
+  // A COPY instruction can be deleted or changed by other optimizations.
+  // Check if the previous COPY instruction is existing and still a COPY.
+  if (!LocalMIs.count(PrevCopy) || !PrevCopy->isCopy())
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qcolombet wrote:

Here we already stretch the use of the `LocalMIs` structure to know if something has been changed and now i feel we abuse the `CopyMIs` structure since it can hold (stale) reference to non copy.

The patch itself is fine as a quick fix, but maybe we'll want to revisit to properly clean up `CopyMIs` (using the delegate mechanism for instance) and don't require any of these two checks here:
- What is in CopyMIs should not have been deleted or should not be a non-copy.

https://github.com/llvm/llvm-project/pull/69511