Speed up by almost 3%

This apparentely silly tweak allows
to speed up the bench by almost 3%.

Not clear why, repeating with perft,
the speed up vanishes.

Suggested by Jonathan Calovski.

No functional change.

src/position.cpp

@@ -821,7 +821,7 @@ void Position::do_move(Move m, StateInfo& newSt, const CheckInfo& ci, bool moveI
           k ^= Zobrist::enpassant[file_of(st->epSquare)];
       }
 
-      if (type_of(m) == PROMOTION)
+      else if (type_of(m) == PROMOTION)
       {
           PieceType promotion = promotion_type(m);