Less NMP if the position was previously in PV.

The intention of the patch is to avoid aggressive null move pruning (NMP)
in positions that have previously been found to be important (PV nodes).
If we already do not apply NMP for current PV nodes, it makes sense to apply
it less often for positions that have previously been PV nodes too.

STC:
LLR: 2.96 (-2.94,2.94) {-1.00,3.00}
Total: 14959 W: 2921 L: 2782 D: 9256
Ptnml(0-2): 254, 1679, 3493, 1762, 282
http://tests.stockfishchess.org/tests/view/5e2f6637ab2d69d58394fcfd

LTC:
LLR: 2.95 (-2.94,2.94) {0.00,2.00}
Total: 6442 W: 899 L: 753 D: 4790
Ptnml(0-2): 42, 549, 1885, 659, 61
http://tests.stockfishchess.org/tests/view/5e2f767bab2d69d58394fd04

closes https://github.com/official-stockfish/Stockfish/pull/2525

Bench: 4725546

src/search.cpp

@@ -841,7 +841,7 @@ namespace {
         && (ss-1)->statScore < 23397
         &&  eval >= beta
         &&  eval >= ss->staticEval
-        &&  ss->staticEval >= beta - 32 * depth + 292 - improving * 30
+        &&  ss->staticEval >= beta - 32 * depth - 30 * improving + 120 * ttPv + 292
         && !excludedMove
         &&  pos.non_pawn_material(us)
         && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))