[llvm] [LoopInterchange] Reject interchange if non-reassociative reduction exists (PR #148612)

[Sjoerd Meijer via llvm-commits]

https://github.com/sjoerdmeijer commented:

This approach is also what I had mind at the end of last week when we discussed this case, so the approach looks good to me. Two nits on FIXMEs inlined. 

https://github.com/llvm/llvm-project/pull/148612