Unifies a bit further the three refuation stages in the MovePicker
class. Also treat the skipping of TT move now always via select_move(),
as discussed in pull request #1454.
Passed STC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 16608 W: 3461 L: 3331 D: 9816
http://tests.stockfishchess.org/tests/view/
5ab0aaf00ebc59029fb6f6c3
Closes https://github.com/official-stockfish/Stockfish/pull/1502
No functional change.