[llvm] [Aarch64] [ISel] Don't save vaargs registers if vaargs are unused (PR #126780)

[Reid Kleckner via llvm-commits]

rnk wrote:

I actually don't know the win64 arm vararg convention, but historically Microsoft has taken an approach much more similar to Apple, where variadic arguments all get pass in memory on the stack, for simplicity, if that helps explain the failures.

https://github.com/llvm/llvm-project/pull/126780