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

[Kristof Beyls via llvm-commits]

https://github.com/kbeyls commented:

I think this patch mostly looks good.
I have one question to verify that I'm understanding the code correctly; and 1 very minor suggestion for an improvement of the name of one of the added tests.

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