Change BDD zero/one node root from -2/-1 to -1/-2

The zero node originally used root = -2 because
0 < 1 and -2 < -1.

The (32-bit) two's complement of -1 is 0xFFFF,
and the (32-bit) two's complement of -2 is 0xFFFE.
Since ~0xFFFF = 0, and ~0xFFFE = 1,
there's a nice little bit of symmetry here.