[llvm] [SLP] Check for extracts, being replaced by original scalars, for user nodes (PR #149572)

[Gaëtan Bossu via llvm-commits]

gbossu wrote:

Just in case you were expecting a review from me, as I wrote [in this comment](https://github.com/llvm/llvm-project/pull/149572#pullrequestreview-3087285610), I don't feel comfortable approving the change because I don't completely understand what it is doing. I'll let the usual experts do the actual thorough review. Sorry if that was not clear :/

https://github.com/llvm/llvm-project/pull/149572