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

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

`build/bin/llc -mtriple=aarch64-unknown-linux-gnu < llvm/test/CodeGen/X86/uscmp.ll` gives a selection error.

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