Simplify optimism scaling formula
Simplify the optimism scaling formula, keeping the i64 cast for safety.
LLR: 2.93 (-2.94,2.94) <-1.75,0.25>
Total: 506336 W: 129447 L: 129738 D: 247151
Ptnml(0-2): 960, 59384, 132718, 59199, 907
Passed STC non-reg:
https://tests.stockfishchess.org/tests/view/6a79ff99a7e815d01b5603ad
Passed LTC non-reg:
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 138276 W: 35651 L: 35552 D: 67073
Ptnml(0-2): 37, 14851, 39261, 14954, 35
https://tests.stockfishchess.org/tests/view/6a842ae29960adfadf924008
closes https://github.com/official-stockfish/Stockfish/pull/7097
Bench: 2497913