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

[Nikita Popov via llvm-commits]

nikic wrote:

This one asserts with `-mattr=+avx512f`:
```llvm
define <33 x i2> @test_ucmp.33.4.2(<33 x i4> %x, <33 x i4> %y) {
  %1 = call <33 x i2> @llvm.ucmp(<33 x i4> %x, <33 x i4> %y)
  ret <33 x i2> %1
}
```

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