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

[via llvm-commits]

Poseydon42 wrote:

Added the tests that were suggested (for AArch64 and x86 only for now, but if any other architectures are suggested I'd be happy to add those too). Also fixed the slightly misleading comment in `WidenVecRes_CMP` and addressed the comment about checking whether operands should also be split in `SplitVecRes_CMP`.

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