[llvm] [X86][CodeGen] Add base atan2 intrinsic lowering (p4) (PR #110760)

[Tex Riddell via llvm-commits]

tex3d wrote:

> GlobalISel test coverage?

Some coverage was added in part 2 earlier [here](https://github.com/llvm/llvm-project/pull/110037/files#diff-1836dbff33b24748f65272b0993967995dea458fc25e36b21a67b305716c0cc5) when global op was added for SPIR-V, but there aren't dedicated MIR tests for SPIR-V.  Since x86_64 doesn't use GlobalISel, we will be testing that a lot more in forthcoming part 5 that adds aarch64 after this merges.

https://github.com/llvm/llvm-project/pull/110760