Remove skipQuiets with mate fix. (official-stockfish#2021)

This removes the skipQuiets variable, as was done in an earlier round by
@protonspring, but fixes an oversight which led to wrong mate
announcements. Quiets can only be pruned when there is no mate score, so
set moveCountPruning at the right spot.

tested as a fix at STC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 66321 W: 14690 L: 14657 D: 36974
http://tests.stockfishchess.org/tests/view/5c74f3170ebc5925cffd4b3c

and as the full patch at LTC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 25903 W: 4341 L: 4228 D: 17334
http://tests.stockfishchess.org/tests/view/5c7540030ebc5925cffd506f

Bench: 3292342

src/search.cpp

@@ -569,7 +569,7 @@ namespace {
     Depth extension, newDepth;
     Value bestValue, value, ttValue, eval, maxValue, pureStaticEval;
     bool ttHit, ttPv, inCheck, givesCheck, improving;
-    bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets, ttCapture;
+    bool captureOrPromotion, doFullDepthSearch, moveCountPruning, ttCapture;
     Piece movedPiece;
     int moveCount, captureCount, quietCount;
 
@@ -879,12 +879,12 @@ namespace {
                                       ss->killers);
     value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
 
-    skipQuiets = false;
+    moveCountPruning = false;
     ttCapture = ttMove && pos.capture_or_promotion(ttMove);
 
     // Step 12. Loop through all pseudo-legal moves until no moves remain
     // or a beta cutoff occurs.
-    while ((move = mp.next_move(skipQuiets)) != MOVE_NONE)
+    while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
     {
       assert(is_ok(move));
 
@@ -913,9 +913,6 @@ namespace {
       movedPiece = pos.moved_piece(move);
       givesCheck = gives_check(pos, move);
 
-      moveCountPruning =   depth < 16 * ONE_PLY
-                        && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
-
       // Step 13. Extensions (~70 Elo)
 
       // Singular extension search (~60 Elo). If all moves but one fail low on a
@@ -966,16 +963,17 @@ namespace {
           && pos.non_pawn_material(us)
           && bestValue > VALUE_MATED_IN_MAX_PLY)
       {
+          // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
+          moveCountPruning = depth < 16 * ONE_PLY
+                          && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
+
           if (   !captureOrPromotion
               && !givesCheck
               && !pos.advanced_pawn_push(move))
           {
               // Move count based pruning (~30 Elo)
               if (moveCountPruning)
-              {
-                  skipQuiets = true;
                   continue;
-              }
 
               // Reduced depth of the next LMR search
               int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;