Adjust singular LMR for positions seen in PV

This patch continues work on altering search for ttPv nodes, using recent idea to alter it more in not PvNodes. Previous tweak based on this idea adjusted singularBeta - this one adjusts value of singularLMR, so they are both related to singular extension search.

passed STC
http://tests.stockfishchess.org/tests/view/5e700737e42a5c3b3ca2e659
LLR: 2.95 (-2.94,2.94) {-0.50,1.50}
Total: 140608 W: 27053 L: 26659 D: 86896
Ptnml(0-2): 2425, 16337, 32439, 16625, 2478

passed LTC
http://tests.stockfishchess.org/tests/view/5e7068eae42a5c3b3ca2e687
LLR: 2.94 (-2.94,2.94) {0.25,1.75}
Total: 79318 W: 10463 L: 10064 D: 58791
Ptnml(0-2): 567, 7416, 23359, 7685, 632

closes official-stockfish#2588

Bench: 4952322

src/search.cpp

@@ -1149,7 +1149,7 @@ namespace {
 
           // Decrease reduction if ttMove has been singularly extended (~3 Elo)
           if (singularLMR)
-              r -= 2;
+              r -= 1 + (ttPv && !PvNode);
 
           if (!captureOrPromotion)
           {