Fix avx512icl update_piece_threats bug

Thanks to MinetaS for discovering this and finding the culprit commit,
specific to this architecture. 9f42980 was subtly incorrect, leading to
spurious wrong benches on avx512icl. Because this piece of code has twice been
the source of wrong benches, I also added a brief comment explaining why it
needs to be this way, and added a sample position to bench.

closes https://github.com/official-stockfish/Stockfish/pull/6654

Bench: 2288704

src/position.cpp

@@ -1131,7 +1131,9 @@ void Position::update_piece_threats(Piece                     pc,
     if constexpr (PutPiece)
     {
         dts->threatenedSqs |= threatened;
-        dts->threateningSqs |= s;
+        // A bit may only be set if that square actually produces a threat, so we
+        // must guard setting the square accordingly
+        dts->threateningSqs |= Bitboard(bool(threatened)) << s;
     }
 
     DirtyThreat dt_template{pc, NO_PIECE, s, Square(0), PutPiece};
@@ -1142,7 +1144,7 @@ void Position::update_piece_threats(Piece                     pc,
 
     if constexpr (PutPiece)
     {
-        dts->threatenedSqs |= s;
+        dts->threatenedSqs |= Bitboard(bool(all_attackers)) << s;  // same as above
         dts->threateningSqs |= all_attackers;
     }