aco: remove some missing label resets

In the case of:

   c = xor(a, b)
   d = not(c)
   xor(d, e)

it will be optimized to:

   d = xnor(a, b)
   xor(d, e)

because "d" would still had a label with "instr=not(c)", it would then be further optimized to:

   d = xnor(a, b)
   xnor(c, e)