Replace depth increase condition with !opponentWorsening

Passed simplification STC
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 220544 W: 57417 L: 57399 D: 105728
Ptnml(0-2): 816, 26554, 55540, 26520, 842
https://tests.stockfishchess.org/tests/view/678970e38082388fa0cbfe02

Passed simplification LTC
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 132600 W: 33868 L: 33760 D: 64972
Ptnml(0-2): 126, 14770, 36390, 14898, 116
https://tests.stockfishchess.org/tests/view/678accabc00c743bc9e9fc7f

closes official-stockfish#5798

bench 1632964

Author: vondele

src/search.cpp

@@ -781,10 +781,8 @@ Value Search::Worker::search(
 
     opponentWorsening = ss->staticEval + (ss - 1)->staticEval > 2;
 
-    if (priorReduction >= 3 && ss->staticEval + (ss - 1)->staticEval < 0)
-    {
+    if (priorReduction >= 3 && !opponentWorsening)
         depth++;
-    }
 
     // Step 7. Razoring (~1 Elo)
     // If eval is really low, skip search entirely and return the qsearch value.