[llvm] [AArch64] Break up `AArch64FrameLowering::emitPrologue` (NFCI) (PR #157485)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

> I think organizing like this makes sense. LGTM. (Note this conflicts with #156467.)

:+1:  I've rebased #156467 locally; the conflicts are fairly minor, so I'll go ahead with this patch.

https://github.com/llvm/llvm-project/pull/157485