prefetch the TT entry as soon as the move is known

Worker::do_move computes the successor hash key via the new
Position::key_after(m) and prefetches the TT entry one full do_move
earlier than the existing prefetch in Position::do_move. key_after does
not model castling, en passant or promotion keys exactly; for rare
moves the prefetch lands on an unused line.

`key_after` has been around since 2014 (https://github.com/official-stockfish/Stockfish/commit/82d065b0) and was removed in (https://github.com/official-stockfish/Stockfish/pull/5770). Adding back `prefetch_key` helps in common, normal moves at the cost of extra compute.

Speedup (PGO vs PGO, interleaved paired bench, n=48 pairs, Apple M2
Pro / apple-silicon): +0.69% [0.47, 0.91]

Passed STC:
https://tests.stockfishchess.org/tests/view/6a291f8d7c758d82accea17f
LLR: 4.24 (-2.94,2.94) <0.00,2.00>
Total: 473504 W: 121250 L: 120228 D: 232026
Ptnml(0-2): 1112, 51137, 131251, 52121, 1131

No functional change

closes https://github.com/official-stockfish/Stockfish/pull/6911

No functional change

AUTHORS

@@ -62,6 +62,7 @@ Clemens L. (rn5f107s2)
 Cody Ho (aesrentai)
 CSTENTOR
 Dale Weiler (graphitemaster)
+Dalton Hanaway (dhanaway)
 Daniel Axtens (daxtens)
 Daniel Dugovic (ddugovic)
 Daniel Monroe (daniel-monroe)