Function round_first 

fn round_first<'tcx, F: Float>(
    ecx: &mut MiriInterpCx<'tcx>,
    left: &OpTy<'tcx>,
    right: &OpTy<'tcx>,
    rounding: &OpTy<'tcx>,
    dest: &MPlaceTy<'tcx>,
) -> InterpResult<'tcx, ()>