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

[Mikhail Gudim via llvm-commits]

mgudim wrote:

Moved this to vector combine.
@RKSimon I haven't implemented the transformation to reduction of concat of two vectors. I can do this in a separate commit.
Also, I left the test where it is now in order to show the effect of the transformation in the diff. I can move it under test/Transforms/VectorCombine in a separate commit.

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