[llvm] Update preserve_most to treat XMM registers like C (PR #73866)

[Seth Brenith via llvm-commits]

sethbrenith wrote:

Thanks for reviewing, @rnk! Would you please advise on the next steps, since I'm unfamiliar with the code review process for LLVM? A few specific questions:

* Would you like me to wait for reviews from others?
* How can I trigger a new buildkite run? Other pull requests from about the same time encountered the same failure, so I assume it's not related to my change.
* Who completes pull requests? It looks like I don't have permission to do so.

https://github.com/llvm/llvm-project/pull/73866