Non-functional speedup: no need to generate, score, or sort quiet moves
if SkipQuiet is true. Thanks to @mstembera for his suggestion.
STC:
LLR: 2.95 (-2.94,2.94) [0.00,4.00]
Total: 27910 W: 6406 L: 6129 D: 15375
http://tests.stockfishchess.org/tests/view/
5d07e0920ebc5925cf0a58a8
Closes https://github.com/official-stockfish/Stockfish/pull/2194
No functional change