[llvm] [InstCombine] Fold binary op of reductions. (PR #121567)

[Mikhail Gudim via llvm-commits]

mgudim wrote:

@davemgreen Sorry, I am still not sure how to proceed. 

(1) Should I close this PR?

or  

(2) 
> We could also consider this to be the canonical pattern and undo the transform in the backend if needed and we could do so reliably.

or 

(3) Move this to vector combine and guard it by target hook?

I prefer (2) because this looks more canonical to me.

https://github.com/llvm/llvm-project/pull/121567