Simplify PV node reduction
Reduce less on PV nodes even with an upperbound TT entry.
Passed STC:
https://tests.stockfishchess.org/tests/view/65cb3a861d8e83c78bfd0497
LLR: 2.96 (-2.94,2.94) <-1.75,0.25>
Total: 118752 W: 30441 L: 30307 D: 58004
Ptnml(0-2): 476, 14179, 29921, 14335, 465
Passed LTC:
https://tests.stockfishchess.org/tests/view/65cd3b951d8e83c78bfd2b0d
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 155058 W: 38549 L: 38464 D: 78045
Ptnml(0-2): 85, 17521, 42219, 17632, 72
closes https://github.com/official-stockfish/Stockfish/pull/5057
Bench: 1303971