Do not remove unused functions annotated with [@zero_alloc]

Do not eliminate functions annotated with `[@zero_alloc]` attributes introduce in https://github.com/ocaml-flambda/flambda-backend/pull/1157, because these annotations are checked in the backend, see example in https://github.com/ocaml-flambda/flambda-backend/pull/1157#issuecomment-1488202057.