Movepicker: take move's loop out of switch statement

Movepicker: take move's loop out of switch statement

This not only cleans up the code but gives another
speed boost of 1.8%

From revision 595a90dfd0 we have increased pgo compiled binary
speed of a whopping +5.2% without any functional change !!

This is really awsome considering that we have also
cut line count by 25 lines.

Sometime we spend days for getting an extra 1% from move
generation while instead the biggest optimizations come
from anonymous and apparently dull parts of the code.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>