Micro-optimize pop_lsb() for 64bit case

On Intel, perhaps due to 'lea' instruction this way of
zeroing the lsb of *b seems faster than a shift+negate.

On perft (where any speed difference is magnified) I
got a 6% speed up on my Intel i5 64bit.

Suggested by Hongzhi Cheng.

No functional change.

src/bitboard.h

@@ -280,7 +280,7 @@ FORCE_INLINE Square msb(Bitboard b) {
 
 FORCE_INLINE Square pop_lsb(Bitboard* b) {
   const Square s = lsb(*b);
-  *b &= ~(1ULL << s);
+  *b &= *b - 1;
   return s;
 }