[llvm] [AArch64] Fix tryMergeAdjacentSTG function in PrologEpilog pass (PR #68873)

[Kristof Beyls via llvm-commits]

https://github.com/kbeyls commented:

I just have a few minor comments mostly on variable naming; otherwise LGTM.

https://github.com/llvm/llvm-project/pull/68873