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

[Maksim Levental via llvm-commits]

makslevental wrote:

This commit (...somehow...) broke Triton/PyTorch on CUDA (see https://github.com/triton-lang/triton/pull/5591 for a description). 

I have attached the failing and passing LLIR:


[failing_triton_poi_fused_33.llir.txt](https://github.com/user-attachments/files/18391665/failing_triton_poi_fused_33.llir.txt)

[passing_triton_poi_fused_33.llir.txt](https://github.com/user-attachments/files/18391661/passing_triton_poi_fused_33.llir.txt)

@alexey-bataev any guidance would be appreciated (I am not familiar with the SLP vectorizer....).

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