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

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

> I wonder if it's worth pulling the vectoriser changes out into a follow-on PR? That way you at least get to keep most of the changes upstream if the vectoriser bits get reverted for some reason.

Sure, I've moved the LV changes to https://github.com/llvm/llvm-project/pull/128035/commits/21b534d9cd8b0a5b7c99272b64e59a9f3fa9d5e6.

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