Split branches in generate_piece_moves()

Split branches in generate_piece_moves()

Instead of one comparison in while() condition use two,
the first to check if the piece is exsistant and the second
to loop across pieces of that type.

This should help branch prediction in cases we have only
one piece of the same type, for instance for queens, the
first branch is always true and the second is almost always
false.

Increased speed of 0.3-0.5 % on Gcc pgo compiles.

No functional change.

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