Tweak late move reductions at root

Tweak late move reductions at root

More LMR at root, unless a fail low might happen.

passed STC:
 LLR: 2.94 (-2.94,2.94) {-1.00,3.00}
Total: 25428 W: 4960 L: 4789 D: 15679
Ptnml(0-2): 424, 2948, 5832, 3045, 460
http://tests.stockfishchess.org/tests/view/5e1c9afed12216a2857e6401

passed LTC:
 LLR: 2.94 (-2.94,2.94) {0.00,2.00}
Total: 187423 W: 24253 L: 23599 D: 139571
Ptnml(0-2): 1284, 17437, 55536, 18085, 1292
http://tests.stockfishchess.org/tests/view/5e1ceb9975be933c8fe635a3

Closes https://github.com/official-stockfish/Stockfish/pull/2493

Bench: 5156767