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

[Maksim Levental via llvm-commits]

makslevental wrote:

@alexey-bataev here are the logs from `print-after-all` for the previous commit (passing) and this commit (failing):

[log_passing.txt](https://github.com/user-attachments/files/18400623/log_passing.txt)
[log_failing.txt](https://github.com/user-attachments/files/18400616/log_failing.txt)





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