Revert hidden checkers rework

It is slower the previous uglier but faster code.

So completely restore old one for now :-(

Just leave in the rework of status backup/restore in do_move().

We will cherry pick bits of previous work once we are sure
we have fixed the performance regression.

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

src/movepick.h

@@ -69,6 +69,7 @@ public:
   int number_of_moves() const;
   int current_move_score() const;
   MovegenPhase current_move_type() const;
+  Bitboard discovered_check_candidates() const;
 
   static void init_phase_table();
 
@@ -83,6 +84,7 @@ private:
 
   const Position& pos;
   Move ttMove, mateKiller, killer1, killer2;
+  Bitboard pinned, dc;
   MoveStack moves[256], badCaptures[64];
   bool pvNode;
   Depth depth;
@@ -107,4 +109,12 @@ inline int MovePicker::number_of_moves() const {
   return numOfMoves;
 }
 
+/// MovePicker::discovered_check_candidates() returns a bitboard containing
+/// all pieces which can possibly give discovered check.  This bitboard is
+/// computed by the constructor function.
+
+inline Bitboard MovePicker::discovered_check_candidates() const {
+  return dc;
+}
+
 #endif // !defined(MOVEPICK_H_INCLUDED)