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

[David Green via llvm-commits]

https://github.com/davemgreen commented:

This will likely cause performance issues unless they are mitigated. There is a combine in the backend that does that same thing, but it gets guarded by shouldReassociateReduction and being in the backend allows the target to pattern recognize before it gets reassociated.

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