Reduce depth after score improvement at PV nodes

STC:
LLR: 2.95 (-2.94,2.94) <0.00,2.50>
Total: 73760 W: 19590 L: 19244 D: 34926
Ptnml(0-2): 285, 8352, 19292, 8634, 317
https://tests.stockfishchess.org/tests/view/626eb2dc9116b52aa83b73da

LTC:
LLR: 2.93 (-2.94,2.94) <0.50,3.00>
Total: 114400 W: 30561 L: 30111 D: 53728
Ptnml(0-2): 68, 11432, 33785, 11812, 103
https://tests.stockfishchess.org/tests/view/626f730859e9c431e0b10b21

closes official-stockfish#4008

bench: 6174823

src/search.cpp

@@ -1300,7 +1300,18 @@ namespace {
                   update_pv(ss->pv, move, (ss+1)->pv);
 
               if (PvNode && value < beta) // Update alpha! Always alpha < beta
+              {
                   alpha = value;
+
+                  // Reduce other moves if we have found at least one score improvement
+                  if (   depth > 2
+                      && depth < 7
+                      && beta  <  VALUE_KNOWN_WIN
+                      && alpha > -VALUE_KNOWN_WIN)
+                     depth -= 1;
+
+                  assert(depth > 0);
+              }
               else
               {
                   ss->cutoffCnt++;