[fb.c] further improvement in is_prime_power_fast
[fb.c] further improvement in is_prime_power_fast

mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs