[clang] [llvm] [AArch64][SelectionDAG] Add CodeGen support for scalar FEAT_CPA (PR #105669)

[Alexander Richardson via cfe-commits]

arichardson wrote:

> Hi @rgwott, I'm working on changes in the AMDGPU backend that would benefit from the PTRADD SelectionDAG opcode that this PR adds. I understand that this PR is low priority for you right now, so I'd like to suggest extracting the relevant changes to enable PTRADD nodes (without the ARM/CPA-specific code) into a separate PR so that we can get them merged sooner. Would you be okay with that? If so, I can create a new PR with the extracted changes (setting you as the author for the commit, so that your work is credited).

If you would like to credit the original author for the PTRADD nodd that would be @davidchisnall (and probably some parts by @jrtc27, although that might just be the combines). I don't see any problems in landing that part separately 

https://github.com/llvm/llvm-project/pull/105669