[llvm] [RISCV] Select atomic_{load/store} to pseudos and expand them later (PR #67108)

[Jessica Clarke via llvm-commits]

jrtc27 wrote:

Do other architectures use this? Having custom pseudo-based lowering for something SDAG can represent directly just fine is pretty sad (and adds more work for us downstream, too), it's a bunch of code that doesn't really need to exist. I would much rather see the generic lowering be adapted to support the optimisations you want (if they are in fact important) rather than making things clunkier like this. I won't _block_ the patch, but I will heavily encourage this change to _not_ be applied if at all possible.

https://github.com/llvm/llvm-project/pull/67108