fn branches<'tcx>(
    ecx: &InterpCx<'tcx, 'tcx, CompileTimeInterpreter<'tcx, 'tcx>>,
    place: &MPlaceTy<'tcx>,
    n: usize,
    variant: Option<VariantIdx>,
    num_nodes: &mut usize
) -> Result<ValTree<'tcx>, ValTreeCreationError>