Speedup and simplify pinners and blockers

To compute dicovered check or pinned pieces we use some bitwise
operators that are not really needed because already accounted for
at the caller site.

For instance in evaluation we compute:

     pos.pinned_pieces(Us) & s

Where pinned_pieces() is:

     st->blockersForKing[c] & pieces(c)

So in this case the & operator with pieces(c) is useless,
given the outer '& s'.

There are many places where we can use the naked blockersForKing[]
instead of the full pinned_pieces() or discovered_check_candidates().

This path is simpler than original and gives around 1% speed up for me.
Also tested for speed by mstembera and snicolet (neutral in both cases).

No functional change.

src/evaluate.cpp

@@ -306,7 +306,7 @@ namespace {
           : Pt ==   ROOK ? attacks_bb<  ROOK>(s, pos.pieces() ^ pos.pieces(QUEEN) ^ pos.pieces(Us, ROOK))
                          : pos.attacks_from<Pt>(s);
 
-        if (pos.pinned_pieces(Us) & s)
+        if (pos.blockers_for_king(Us) & s)
             b &= LineBB[pos.square<KING>(Us)][s];
 
         attackedBy2[Us] |= attackedBy[Us][ALL_PIECES] & b;
@@ -392,8 +392,8 @@ namespace {
         if (Pt == QUEEN)
         {
             // Penalty if any relative pin or discovered attack against the queen
-            Bitboard pinners;
-            if (pos.slider_blockers(pos.pieces(Them, ROOK, BISHOP), s, pinners))
+            Bitboard queenPinners;
+            if (pos.slider_blockers(pos.pieces(Them, ROOK, BISHOP), s, queenPinners))
                 score -= WeakQueen;
         }
     }
@@ -413,7 +413,7 @@ namespace {
                                        : AllSquares ^ Rank1BB ^ Rank2BB ^ Rank3BB);
 
     const Square ksq = pos.square<KING>(Us);
-    Bitboard weak, b, b1, b2, safe, unsafeChecks;
+    Bitboard weak, b, b1, b2, safe, unsafeChecks, pinned;
 
     // King shelter and enemy pawns storm
     Score score = pe->king_safety<Us>(pos, ksq);
@@ -464,11 +464,12 @@ namespace {
         // Unsafe or occupied checking squares will also be considered, as long as
         // the square is in the attacker's mobility area.
         unsafeChecks &= mobilityArea[Them];
+        pinned = pos.blockers_for_king(Us) & pos.pieces(Us);
 
         kingDanger +=        kingAttackersCount[Them] * kingAttackersWeight[Them]
                      + 102 * kingAdjacentZoneAttacksCount[Them]
                      + 191 * popcount(kingRing[Us] & weak)
-                     + 143 * popcount(pos.pinned_pieces(Us) | unsafeChecks)
+                     + 143 * popcount(pinned | unsafeChecks)
                      - 848 * !pos.count<QUEEN>(Them)
                      -   9 * mg_value(score) / 8
                      +  40;