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

[David Spickett via llvm-commits]

DavidSpickett wrote:

Linaro's 2 stage Windows arm64 build also started failing on a build including this: https://lab.llvm.org/buildbot/#/builders/124/builds/490

(I made a mistake analysing the results, that's why we're delayed so much)

We are investigating on our side.

(FYI @luporl)

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