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

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> 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.

Agree.

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