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

[Tex Riddell via llvm-commits]

tex3d wrote:

Switching to draft to sort this out.  I will either try to land the aarch64 part first, or if that's too difficult, I'll strip the GlobalISel changes from this one.

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