[llvm] [SLP] Match poison as instruction with the same opcode (PR #115946)

[Maksim Levental via llvm-commits]

makslevental wrote:

> Try to check the latest head. I don't quite understand what is the problem. What shall I do to reproduce the issue?

I've tried - same error. The problem is that somehow this PR causes illegal accesses:

```
Traceback (most recent call last):
  File "/home/mleventa/dev_projects/repro/repro.py", line 135, in <module>
    torch.cuda.synchronize()
  File "/home/mleventa/dev_projects/.venv/lib/python3.10/site-packages/torch/cuda/__init__.py", line 987, in synchronize
    return torch._C._cuda_synchronize()
RuntimeError: CUDA error: an illegal memory access was encountered
```

Here are some possibly incorrect emitted instructions:

Passing <-------------> Failing
![image](https://github.com/user-attachments/assets/e07d3244-6939-4550-b703-c15f9d7ff169)
Notice that prior the global store had non-zero operands and now (on this commit) the same store receives all `0`s

Passing <-------------> Failing
![image](https://github.com/user-attachments/assets/2ca1ceb1-33da-4a35-b928-092e0082d6e7)
Notice that prior the `mov` received a non-zero operand but now receives poison.

I have not fully debugged.




https://github.com/llvm/llvm-project/pull/115946