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

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

Thanks for the reviews! I've got the final patch for `llvm.sincos` here https://github.com/llvm/llvm-project/pull/128035, which is a trivial patch which just enables it's vectorization. 

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