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

[Alexey Bataev via llvm-commits]

alexey-bataev 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

This one is Dec 30, I fixed a bug earlier today

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