[clang] [clang][CodeGen] Emit atomic IR in place of optimized libcalls. (PR #73176)

[Alexander Richardson via cfe-commits]

https://github.com/arichardson commented:

Overall this looks great to me, very happy to see reduced duplication between clang and llvm.

It would be great if we could just emit the atomic IR in all cases but unfortunately non-power-of-two sizes result in a verifier error. We could probably emit the atomic IR for power-of-two sizes and let the expand pass deal with it but since we still need the fallback for other sizes we might as well keep it.

https://github.com/llvm/llvm-project/pull/73176