Micro optimize and rename move_promotion()

Rename to move_is_promotion() to be more clear, also add
a new function move_promotion_piece() to get the
promotion piece type in the few places where is needed.

No functional change.

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

src/move.h

@@ -78,10 +78,14 @@ inline Square move_to(Move m) {
   return Square(m & 0x3F);
 }
 
-inline PieceType move_promotion(Move m) {
+inline PieceType move_promotion_piece(Move m) {
   return PieceType((int(m) >> 12) & 7);
 }
 
+inline int move_is_promotion(Move m) {
+  return m & (7 << 12);
+}
+
 inline int move_is_ep(Move m) {
   return m & (1 << 15);
 }