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

[Hans Wennborg via llvm-commits]

zmodem wrote:

We're hitting a large amount of test failures on Windows arm64 after this change.

+cc @rnk for windows and calling convention expertise. I don't really understand what "Don't save vaargs registers if vaargs are unused" means, but maybe Windows is different somehow?

For example, in the changed file I see several instances of `if (IsWin64) <do something based on isVarArg>`, and this PR is only changing *one* use of `isVarArg`, so maybe things were getting out of sync?

I'll prepare a revert of this for now.

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