t_limb</code> on various processors might benefit from the
     little Newton iteration done for alpha and ia64.
<li> Alpha 21264: <code>mpn_addlsh1_n</code> could be implemented with
     <code>mpn_addmul_1</code>, since that code at 3.5 is a touch faster than
     a separate <code>lshift</code> and <code>add_n</code> at
     1.75+2.125=3.875.  Or very likely some specific <code>addlsh1_n</code>
     code could beat both.
<li> Alpha 21264: Improve feed-in code for <code>mpn_mul_1</code>,
     <code>mpn_add