In this patch x_mul(a, b) uses fewer bit operations for a != b,
asymptotically half of them.
On the three computers I tried the speed-up is around 5% for size=4
and it increases up to 45-60% just below the Karatsuba cutoff,
then it decreases a bit after this cutoff (on one computer the speed-up
is only 16% after KARATSUBA_CUTOFF=70, but raising the cutoff to 140,
for which with the current code the multiplication is also faster,
the speed-up is 45%).