Simplifies bestMoveChanges from LMR

Simplifies bestMoveChanges from LMR

As bestMoveChanges is only reset on mainThread and it could change how other
threads search, a multi-threads test was made.

STC:
LLR: 2.94 (-2.94,2.94) <-2.25,0.25>
Total: 146776 W: 37934 L: 37941 D: 70901
Ptnml(0-2): 477, 15644, 41173, 15597, 497
https://tests.stockfishchess.org/tests/view/61a8f9f34ed77d629d4ea2d6

LTC:
LLR: 3.11 (-2.94,2.94) <-2.25,0.25>
Total: 114040 W: 29314 L: 29269 D: 55457
Ptnml(0-2): 50, 10584, 35722, 10599, 65
https://tests.stockfishchess.org/tests/view/61a9d4bf9e8855bba1a35c4f

(SMP, 8 threads) STC:
LLR: 2.95 (-2.94,2.94) <-2.25,0.25>
Total: 23888 W: 6308 L: 6143 D: 11437
Ptnml(0-2): 36, 2557, 6600, 2708, 43
https://tests.stockfishchess.org/tests/view/61ac27a756fcf33bce7d3677

closes https://github.com/official-stockfish/Stockfish/pull/3831

bench: 4829419