[llvm] [SelectionDAG][x86] Ensure vector reduction optimization (PR #144231)

[Suhajda Tamás via llvm-commits]

sutajo wrote:

I agree that it makes little sense to expand the reductions and then later recombine them. 
Alright, I'll look into lowering some reduction OPs in the x86 backend.

https://github.com/llvm/llvm-project/pull/144231