fn codegen_autodiff<'ll, 'tcx>(
bx: &mut GenericBuilder<'_, 'll, FullCx<'ll, 'tcx>>,
instance: Instance<'tcx>,
args: &[OperandRef<'tcx, Value<'ll>>],
result_layout: TyAndLayout<'tcx>,
result_place: Option<PlaceValue<Value<'ll>>>,
) -> IntrinsicResult<'tcx, Value<'ll>>