Decrease reductions in Lmr for some Pv nodes

This patch makes us reduce less in Lmr at pv nodes in case of static eval being far away from static evaluation of position.
Idea is that if it's the case then probably position is pretty complex so we can't be sure about how reliable LMR is so we need to reduce less.

Passed STC:
https://tests.stockfishchess.org/tests/view/6226276aa9d47c8160e81220
LLR: 2.94 (-2.94,2.94) <0.00,2.50>
Total: 262696 W: 69944 L: 69239 D: 123513
Ptnml(0-2): 1399, 29702, 68436, 30417, 1394

Passed LTC:
https://tests.stockfishchess.org/tests/view/6226b002a9d47c8160e82b91
LLR: 2.95 (-2.94,2.94) <0.50,3.00>
Total: 64008 W: 17320 L: 16982 D: 29706
Ptnml(0-2): 60, 6378, 18811, 6674, 81

closes https://github.com/official-stockfish/Stockfish/pull/3957

bench 6678390

src/search.cpp

@@ -1166,6 +1166,11 @@ moves_loop: // When in check, search starts here
           if (ttCapture)
               r++;
 
+          // Decrease reduction at PvNodes if bestvalue
+          // is vastly different from static evaluation
+          if (PvNode && !ss->inCheck && abs(ss->staticEval - bestValue) > 250)
+              r--;
+
           ss->statScore =  thisThread->mainHistory[us][from_to(move)]
                          + (*contHist[0])[movedPiece][to_sq(move)]
                          + (*contHist[1])[movedPiece][to_sq(move)]