Reduction instead of cutoff

[snicolet]

In master, during singular move analysis, when both the transposition value
and a reduced search for the other moves seem to indicate a fail high, we
heuristically prune the whole subtree and return an fail high score.

This patch is a little bit more cautious in this case, and instead of the
risky cutoff, we now search the ttMove with a reduced depth (by two plies).

STC:
https://tests.stockfishchess.org/tests/view/614dafe07bdc23e77ceb8a89
LLR: 2.94 (-2.94,2.94) <-0.50,2.50>
Total: 46728 W: 11909 L: 11666 D: 23153
Ptnml(0-2): 181, 5288, 12168, 5561, 166

LTC:
https://tests.stockfishchess.org/tests/view/614dc84abe4c07e0ecac3c95
LLR: 2.94 (-2.94,2.94) <0.50,3.50>
Total: 74520 W: 18809 L: 18450 D: 37261
Ptnml(0-2): 45, 7735, 21346, 8084, 50

closes #3718

Bench: 5499262

[jhellis3]

Congrats! I had noticed a trend of SF generally becoming stronger but less tactically aware over the last several months. This patch not only increases strength but significantly improves tactical awareness in my (limited) testing.

[snicolet]

Congrats! I had noticed a trend of SF generally becoming stronger but less tactically aware over the last several months. This patch not only increases strength but significantly improves tactical awareness in my (limited) testing.

:-)

[Vizvezdenec]

In last year we've done a pretty big refactor of how search tree looks I think - now we do a lot of extending of promising branches compared to what we did previously and this reflects in bench number than increased almost twice.
Doing depth reduction instead of multicut also should affect this a lot, I'm pretty excited for the future :)