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)