[llvm] [AArch64] Fix register reversing crash on MSVC with 1 pair. (PR #82392)

[Momchil Velikov via llvm-commits]

https://github.com/momchil-velikov commented:

I suspect the issue is with dereferencing `PPREnd.base()` when `PPREnd == RegPairs.rend()` (same with `ZPR`).
And it can happen even if `RegPairs.size() > 1`, for example if there are no `PPR` pairs at all.
(it'll be a couple of hours before I get my hands on a Windows machine to confirm and/or investigate).

https://github.com/llvm/llvm-project/pull/82392