[llvm] [SelectionDAG] Add support for the 3-way comparison intrinsics [US]CMP (PR #91871)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

This looks mostly good to me -- main uncertainty is around the vector widening implementations, which are pretty unusual. It would be good if an SDAG expert could look over that part at least (@RKSimon maybe?)

https://github.com/llvm/llvm-project/pull/91871