[llvm] [CostModel] Handle vector struct results and cost `llvm.sincos` (PR #123210)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

Created a follow-up for the flags here: https://github.com/llvm/llvm-project/pull/128885. I've not removed them or changed the default yet, but feel free to discuss that on the new patch. 

https://github.com/llvm/llvm-project/pull/123210