Simplify IIR
Simplified depth reduction for PV nodes without a ttMove to 3.
Passed STC non-reg:
https://tests.stockfishchess.org/tests/view/65d1a90a1d8e83c78bfd855a
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 363168 W: 93648 L: 93791 D: 175729
Ptnml(0-2): 1557, 43692, 91221, 43565, 1549
Passed LTC non-reg:
https://tests.stockfishchess.org/tests/view/65d5612d1d8e83c78bfdc8e2
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 58818 W: 14946 L: 14761 D: 29111
Ptnml(0-2): 36, 6595, 15962, 6780, 36
closes https://github.com/official-stockfish/Stockfish/pull/5062
Bench: 1505827