Make key_after() more consistent with key()
STC: https://tests.stockfishchess.org/tests/view/62f8547123d42b50a8dac674
LLR: 2.95 (-2.94,2.94) <0.00,2.00>
Total: 176640 W: 47699 L: 47189 D: 81752
Ptnml(0-2): 776, 18599, 49129, 18971, 845
A bug fix plus non functional speed optimization. Position::key_after(Move m) is now
consistent with Position::key() thus prefetching correct TT entries which speeds things up.
Related PR #3759
closes https://github.com/official-stockfish/Stockfish/pull/4130
No functional change