[llvm] [SLP] Simplify tryToFindDuplicates() (NFC) (PR #135766)

[Gaëtan Bossu via llvm-commits]

gbossu wrote:

Thanks for the review @alexey-bataev , I have now simplified the PR so it only touches the code in `tryToFindDuplicates()`, and I'll create a follow up for refactoring the legality checks. I think it's pretty small and straight-forward now, let me know what you think :)

Note: I have also updated the PR description.

https://github.com/llvm/llvm-project/pull/135766