Fall back to NNUE if classical evaluation is much lower than threshold

The idea is that if classical eval returns a value much lower than the threshold of
its usage it most likely means that position isn't that simple
so we need the more precise NNUE evaluation.

passed STC:
https://tests.stockfishchess.org/tests/view/61bf3e7557a0d0f327c3c47a
LLR: 2.95 (-2.94,2.94) <0.00,2.50>
Total: 108072 W: 28007 L: 27604 D: 52461
Ptnml(0-2): 352, 12147, 28650, 12520, 367

passed LTC:
https://tests.stockfishchess.org/tests/view/61c0581657a0d0f327c3fa0c
LLR: 2.95 (-2.94,2.94) <0.50,3.00>
Total: 155096 W: 40392 L: 39841 D: 74863
Ptnml(0-2): 88, 15983, 44843, 16558, 76

closes https://github.com/official-stockfish/Stockfish/pull/3869

bench 4310422

src/evaluate.cpp

@@ -1086,10 +1086,17 @@ Value Eval::evaluate(const Position& pos) {
   // Deciding between classical and NNUE eval (~10 Elo): for high PSQ imbalance we use classical,
   // but we switch to NNUE during long shuffling or with high material on the board.
 
+  bool classical = false;
+
   if (  !useNNUE
       || abs(eg_value(pos.psq_score())) * 5 > (850 + pos.non_pawn_material() / 64) * (5 + pos.rule50_count()))
+  {
       v = Evaluation<NO_TRACE>(pos).value();          // classical
-  else
+      classical = abs(v) >= 300;
+  }
+
+  // If result of a classical evaluation is much lower than threshold fall back to NNUE
+  if (!classical && useNNUE)
   {
        int scale = 1136
                    + 20 * pos.non_pawn_material() / 1024;