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

[Maksim Levental via llvm-commits]

makslevental wrote:

> Try to update the compiler to most recent version. I see an incorrect vectorization result in the failed version, which was fixed already.

I/we've tried `91c5de7fb8f95132043ed08056e58238383cfcb9` but sure I can try HEAD

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