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

[Nikita Popov via llvm-commits]

nikic wrote:

It would also be good to add some test coverage for other targets -- for those cases, I don't think we need the full legalization test coverage, but mainly some scalar tests with common types, so we can see how the codegen looks like and have a baseline when improving it later. (The currently used expansion is probably not the best default expansion, but it will be easier to judge that with baseline test coverage.)

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