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

[Antonio Frighetto via llvm-commits]

antoniofrighetto wrote:

@fhahn, sorry for the delay in getting back, and thanks for the ping; this took a bit more than expected. I pushed the test (and variants thereof) in 88d0ceb68936d8905bfc1724a18690b3b6115f94. This seems to reveal that we fail in emitting a `stlurb` and `stlurh` when leveraging GISel ([here](https://github.com/llvm/llvm-project/blob/main/llvm/test/CodeGen/AArch64/Atomics/aarch64-atomic-store-rcpc_immo.ll#L360)). I think https://github.com/antoniofrighetto/llvm-project/commit/f253bfc95eda2fc48e0bef2ed1f49db1ed63dbf2 could solve the missed opportunity, but I’m not sure why we bail out from `selectImpl` this time, for these two instructions. Hope I can take a look at this soon.

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