Skip non-pv research if lmr depth does not reduce

[dallinson]

STC:

Elo   | 2.25 +- 4.30 (95%)
SPRT  | 8.0+0.08s Threads=1 Hash=16MB
LLR   | 2.89 (-2.25, 2.89) [-5.00, 0.00]
Games | N: 12204 W: 3285 L: 3206 D: 5713
Penta | [368, 1454, 2385, 1521, 374]

LTC:

Elo   | 0.74 +- 3.19 (95%)
SPRT  | 40.0+0.40s Threads=1 Hash=64MB
LLR   | 2.90 (-2.25, 2.89) [-5.00, 0.00]
Games | N: 19278 W: 4428 L: 4387 D: 10463
Penta | [424, 2336, 4104, 2325, 450]

Bench: 4102474