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

[Reid Kleckner via llvm-commits]

rnk wrote:

I'm comfortable approving and merging this pull request, so I went ahead and did that.

For LLVM committers, I would generally prefer to let them be the ones to push the merge button, so I didn't do that earlier.

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