Modify ttPV reduction
This patch modifies ttPV reduction by reducing 1 more unless ttValue is above alpha.
Inspired from @pb00068 https://tests.stockfishchess.org/tests/view/658060796a3b4f6202215f1f
Passed STC:
https://tests.stockfishchess.org/tests/view/6591867679aa8af82b958328
LLR: 2.94 (-2.94,2.94) <0.00,2.00>
Total: 37856 W: 9727 L: 9407 D: 18722
Ptnml(0-2): 99, 4444, 9568, 4672, 145
Passed LTC:
https://tests.stockfishchess.org/tests/view/6591d9b679aa8af82b958a6c
LLR: 2.94 (-2.94,2.94) <0.50,2.50>
Total: 128256 W: 32152 L: 31639 D: 64465
Ptnml(0-2): 64, 14364, 34772, 14851, 77
closes https://github.com/official-stockfish/Stockfish/pull/4957
Bench: 1176235