[llvm] [SLP] Simplify buildTree() and callees (NFC) (PR #135766)

[Gaëtan Bossu via llvm-commits]

gbossu wrote:

FYI @alexey-bataev @RKSimon I've seen some of the refactorings made in https://github.com/llvm/llvm-project/commit/02a708b93b9ebe5e4fbc2b266da94677e6f793d3 and https://github.com/llvm/llvm-project/commit/61d04f1aac96f671a3975e04dac02d270b86ac4d and I think they are great, it's a lot easier to follow the code!

I had started some other changes already which go a bit further. They are summarized in the PR description. I've split the changes in multiple commits to make the review easier. I could also create multiple PRs if needed.  I'd be happy to hear your opinion, cheers! 🙂 

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