 If low bits of the low limb are zero, use <code>mpn_rshift</code> so as
     to not increase the size.
<li> <code>mpn_dc_sqrtrem</code>: Don't use <code>mpn_addmul_1</code> with
     multiplier==2, instead either <code>mpn_addlsh1_n</code> when available,
     or <code>mpn_lshift</code>+<code>mpn_add_n</code> if not.
<li> <code>mpn_dc_sqrtrem</code>, <code>mpn_sqrtrem2</code>: Don't use
     <code>mpn_add_1</code> and <code>mpn_sub_1</code> for 1 limb operations,
     instead <code>ADDC_LIMB</code> and <co