factor: omit unnecessary divisions by small primes

factor: omit unnecessary divisions by small primes

* src/factor.c (mp_factor_using_division):
When continuing in single precision, don’t divide by primes that
were already cast out in multiple precision.
On my platform this gave a 2.5% speedup when factoring
2**128 + 172261 = 4999 * 68070087401668026297934508388031283,
as W_TYPE_SIZE == 64 and 4999 is the last prime in the primes table.