LMR search tweak

All credit to Vizvezdenec, the original author of the idea.

STC https://tests.stockfishchess.org/tests/view/5f2d606a61e3b6af64881f88
LLR: 2.95 (-2.94,2.94) {-0.50,1.50}
Total: 8440 W: 1191 L: 1048 D: 6201
Ptnml(0-2): 59, 754, 2467, 865, 75

LTC https://tests.stockfishchess.org/tests/view/5f2d84ad61e3b6af64881fbd
LLR: 2.95 (-2.94,2.94) {0.25,1.75}
Total: 21896 W: 1557 L: 1406 D: 18933
Ptnml(0-2): 33, 1185, 8378, 1302, 50

closes #2951

bench: 4084753

src/search.cpp

@@ -1159,7 +1159,7 @@ namespace {
       // Step 16. Reduced depth search (LMR, ~200 Elo). If the move fails high it will be
       // re-searched at full depth.
       if (    depth >= 3
-          &&  moveCount > 1 + 2 * rootNode
+          &&  moveCount > 1 + 2 * rootNode + 2 * (PvNode && abs(bestValue) < 2)
           && (!rootNode || thisThread->best_move_count(move) == 0)
           && (  !captureOrPromotion
               || moveCountPruning