[llvm] Spaceship intrinsic for GSoC 2024 (PR #83227)

[Miguel Raz Guzmán Macedo via llvm-commits]

miguelraz wrote:

@nikic Alright, first proper draft is up.

Areas for improvement:

1. I didn't know how to bring up the return type for `sthreecmp` or `uthreecmp` to be `iM` bits in the LangRef. I welcome suggestions.
2. Tablegen is new and scary. I searched for similar optims and I added the `IntrSpeculatable` to the block. 
3. I don't have a strong stance on the naming for `sthreecmp`. Bikeshed away.

https://github.com/llvm/llvm-project/pull/83227