[llvm] [SLP][NFC]Extract preliminary checks from buildTree_rec, NFC (PR #134132)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

its very difficult to diff this to check for any unintended changes that could creep in - is there no way we can simplify the changes more?

https://github.com/llvm/llvm-project/pull/134132