Do more reductions at Pv nodes with low delta
This patch increases reduction for PvNodes that have their delta (difference between beta and alpha) significantly reduced compared to what it was at root.
passed STC
https://tests.stockfishchess.org/tests/view/
617f9063af49befdeee40226
LLR: 2.94 (-2.94,2.94) <0.00,2.50>
Total: 220840 W: 55752 L: 55150 D: 109938
Ptnml(0-2): 583, 24982, 58712, 25536, 607
passed LTC
https://tests.stockfishchess.org/tests/view/
61815de959e71df00dcc42ed
LLR: 2.95 (-2.94,2.94) <0.50,3.00>
Total: 79000 W: 19937 L: 19562 D: 39501
Ptnml(0-2): 36, 8190, 22674, 8563, 37
closes https://github.com/official-stockfish/Stockfish/pull/3774
bench:
6717808