[llvm] [InstSimplify] Add basic constant folding for `llvm.sincos` (PR #114527)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

> Could you add something about `afn`? IR and/or summery.

Sure, I don't believe constant folding requires `afn` though, right? :slightly_smiling_face: 

https://github.com/llvm/llvm-project/pull/114527