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

[Tex Riddell via llvm-commits]

tex3d wrote:

I've moved GlobalISel changes to the next aarch64 step.
@RKSimon I hope this satisfies the GlobalISel test concerns for this PR.

I gave up trying to rearrange the steps as the aarch64 change has common SelectionDAG (and other) dependencies on changes in this PR, so aarch64 lowering is still be staged after this one.

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