[llvm] [IR] Add `llvm.sincos` intrinsic (PR #109825)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

> > We've never made any progress vectorizing intrinsics with aggregate return types like this (e.g. overflow intrinsics)
> > @fhahn @alexey-bataev Do you think we can easily support this or should we consider alternatives? We've also had problems with handling arguments that take pointers (sincos / modf etc.) so I guess its just an annoying problem in general.......
> 
> I have a patch WIP/experimental patch to support this, I could share as a draft now.

Here's my (experimental/WIP) implementation for vectorizing (homogeneous) structs: https://github.com/llvm/llvm-project/pull/109833. It works for my current set of test cases (based on functions like sincos).

https://github.com/llvm/llvm-project/pull/109825