[llvm] [BPF] Use FETCH atomic instruction for monotonic atomicrmw when the result is used (PR #210371)

[Gustas Janušonis via llvm-commits]

LeGusto wrote:

@yonghong-song  @eddyz87  this is ready for review when you get a chance, couldn't find an automatic label for BPF

https://github.com/llvm/llvm-project/pull/210371