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

[Maksim Levental via llvm-commits]

makslevental wrote:

@alexey-bataev ah sorry - the initial dumps I pasted weren't at the right point in the pass pipeline; those different stores actually appear at the end of `InstCombinePass`.

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