Less reduction for escape moves at ttPv nodes

At expected PV nodes or nodes which marked as PV node in the hash table,
reduce escape moves even one ply less.

STC:
LLR: 2.94 (-2.94,2.94) {-1.00,3.00}
Total: 31795 W: 6140 L: 5953 D: 19702
Ptnml(0-2): 525, 3625, 7455, 3695, 583
http://tests.stockfishchess.org/tests/view/5e25d77fc3b97aa0d75bc013

LTC:
LLR: 2.94 (-2.94,2.94) {0.00,2.00}
Total: 43975 W: 5708 L: 5454 D: 32813
Ptnml(0-2): 314, 4012, 13070, 4242, 325
http://tests.stockfishchess.org/tests/view/5e2618c1c3b97aa0d75bc03c

Closes https://github.com/official-stockfish/Stockfish/pull/2505

Bench: 4475583

src/search.cpp

@@ -625,7 +625,7 @@ namespace {
     Move ttMove, move, excludedMove, bestMove;
     Depth extension, newDepth;
     Value bestValue, value, ttValue, eval, maxValue;
-    bool ttHit, ttPv, inCheck, givesCheck, improving , didLMR, priorCapture;
+    bool ttHit, ttPv, inCheck, givesCheck, improving, didLMR, priorCapture;
     bool captureOrPromotion, doFullDepthSearch, moveCountPruning, ttCapture, singularLMR;
     Piece movedPiece;
     int moveCount, captureCount, quietCount;
@@ -825,7 +825,6 @@ namespace {
         &&  eval <= alpha - RazorMargin)
         return qsearch<NT>(pos, ss, alpha, beta);
 
-    // (~13 Elo)
     improving =  (ss-2)->staticEval == VALUE_NONE ? (ss->staticEval > (ss-4)->staticEval
               || (ss-4)->staticEval == VALUE_NONE) : ss->staticEval > (ss-2)->staticEval;
 
@@ -1161,7 +1160,7 @@ moves_loop: // When in check, search starts from here
               // hence break make_move(). (~2 Elo)
               else if (    type_of(move) == NORMAL
                        && !pos.see_ge(reverse_move(move)))
-                  r -= 2;
+                  r -= 2 + ttPv;
 
               ss->statScore =  thisThread->mainHistory[us][from_to(move)]
                              + (*contHist[0])[movedPiece][to_sq(move)]