Simplify internal iterative reductions
Passed STC:
LLR: 2.92 (-2.94,2.94) <-1.75,0.25>
Total: 138656 W: 36182 L: 36074 D: 66400
Ptnml(0-2): 523, 16422, 35310, 16570, 503
https://tests.stockfishchess.org/tests/view/6702beb386d5ee47d953bd41
Passed LTC:
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 680844 W: 172021 L: 172480 D: 336343
Ptnml(0-2): 492, 76259, 187419, 75720, 532
https://tests.stockfishchess.org/tests/view/67042b1f86d5ee47d953be7c
closes https://github.com/official-stockfish/Stockfish/pull/5634
Bench: 1169252