[llvm] AArch64: Replace AArch64MCExpr with MCSpecifierExpr (PR #144632)

[Peter Smith via llvm-commits]

https://github.com/smithp35 approved this pull request.

Thanks for the patch. FWIW this looks good to me. Having the AArch64 backend be more similar to the other backends, including ARM should make it easier for new contributors.

I've marked approval from me. While there are a lot of changes, most of the changes are straightforward substitutions. There's a lot of reviewers and stakeholders though, please leave some time for other reviewers to object.

I'll wait for this to merge before going on to https://github.com/llvm/llvm-project/pull/144633 

https://github.com/llvm/llvm-project/pull/144632