Tweak check extension condition

There are two concepts with this patch:

Limit check extensions by using move count.
The idea is to limit search explosion.

Always extend check if the first move gives check.
The idea is to save expensive SEE calls, since the vast
majority of first move will have SEE value >= 0, also
first move may still be strong even if the SEE is negative.

STC:
LLR: 2.95 (-2.94,2.94) [0.00,5.00]
Total: 16503 W: 3068 L: 2873 D: 10562

LTC:
LLR: 2.97 (-2.94,2.94) [0.00,5.00]
Total: 37202 W: 5261 L: 5014 D: 26927

bench: 8543366

src/search.cpp

@@ -608,7 +608,7 @@ namespace {
     Depth extension, newDepth, predictedDepth;
     Value bestValue, value, ttValue, eval, nullValue;
     bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
-    bool captureOrPromotion, doFullDepthSearch;
+    bool captureOrPromotion, doFullDepthSearch, moveCountPruning;
     Piece moved_piece;
     int moveCount, quietCount;
 
@@ -919,8 +919,13 @@ namespace {
                   ? ci.checkSquares[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
                   : pos.gives_check(move, ci);
 
+      moveCountPruning =   depth < 16 * ONE_PLY
+                        && moveCount >= FutilityMoveCounts[improving][depth];
+
       // Step 12. Extend checks
-      if (givesCheck && pos.see_sign(move) >= VALUE_ZERO)
+      if (   givesCheck
+          && (    moveCount == 1
+              || (!moveCountPruning && pos.see_sign(move) >= VALUE_ZERO)))
           extension = ONE_PLY;
 
       // Singular extension search. If all moves but one fail low on a search of
@@ -956,8 +961,7 @@ namespace {
           &&  bestValue > VALUE_MATED_IN_MAX_PLY)
       {
           // Move count based pruning
-          if (   depth < 16 * ONE_PLY
-              && moveCount >= FutilityMoveCounts[improving][depth])
+          if (moveCountPruning)
               continue;
 
           // Countermoves based pruning