Correct reverse complements of CG and AT

6=S=CG and 9=W=AT are each their own complement.
This table can be generated and verified via a small script:

print join(", ", map {($_&1)<<3 | ($_&2)<<1 | ($_&4)>>1 | ($_&8)>>3} 0..15);

Fixes samtools/htslib#5