[llvm] Reapply "[Aarch64] [ISel] Don't save vaargs registers if vaargs are unused" (PR #131459)

[Oliver Stannard via llvm-commits]

https://github.com/ostannard approved this pull request.

LGTM, though we only require one approval, so it would have been fine to merge this after @zmodem's approval last week.

https://github.com/llvm/llvm-project/pull/131459