[llvm] [AArch64][ISel] Add support for v8.4a RCpc `ldapur`/`stlur` (PR #67879)

[Florian Hahn via llvm-commits]

fhahn wrote:

> @fhahn, sorry, just saw this, didn't see the comment. Could the LLVM IR function you provided look good as a test? I assume we want to exercize `isAArch64FrameOffsetLegal` here.

Yes, it exposes at least the crash. Might be good to check if there are any other test gaps.

https://github.com/llvm/llvm-project/pull/67879