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

[Justin Bogner via llvm-commits]

bogner 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.

X86 does have some GlobalISel implemented, but it doesn't look like any handling of trig functions is in place yet. I think we should probably implement the generic parts of this change along with a backend where we can actually test it (like AArch64), and simplify this PR to only be the X86 parts. Since none of the trig is implemented in X86 GlobalISel at this time, we can probably leave that aspect alone.

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