Function rustc_apfloat::ieee::sig::add_or_sub

pub(super) fn add_or_sub(
    a_sig: &mut [u128],
    a_exp: &mut ExpInt,
    a_sign: &mut bool,
    b_sig: &mut [u128],
    b_exp: ExpInt,
    b_sign: bool
) -> Loss

a += b or a -= b. Does not preserve b.