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

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

> Do you want to get this in before #109833 ? Whichever way you do it, I'd like to see sincos vectorisation test coverage.

I've split them so they can go in any order. Just #109833 alone won't vectorize `llvm.sincos`, the vectorizer also needs to be told it's possible (which is just a small patch), so that's when I'm planning on adding sincos vectorization tests. 

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