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

[Sjoerd Meijer via llvm-commits]

sjoerdmeijer wrote:

My feedback and @nikic's crossed, I had not seen his when I pressed the button, so maybe look first into his remarks first. 

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