[llvm] [AArch64] Remove redundant COPY from loadRegFromStackSlot (PR #107396)

[Sander de Smalen via llvm-commits]

sdesmalen-arm wrote:

> Since this was found in the wild I'd also like to see the IR test added to make sure the original case is fixed too.
When I add the IR test to this patch it makes cherry-picking for LLVM 19 harder, because there is then a dependence on #107406. I can push the IR test as a separate NFC patch after merging this PR, are you okay with that?

https://github.com/llvm/llvm-project/pull/107396