How to send a digital signal when analog sensor (QRE1113) reaches a threshold value without resorting to wasting analog input pin?
As mentioned in the comments, you want a comparator.

At a high level, a comparator has two inputs, a positive and a negative, and one output which is high whenever the positive input is at a greater voltage than the negative input. You might notice that this is just like what op amps do when you don't give them any feedback; indeed, the circuitry for a comparator is very similar to that for an op amp, and you can use an op amp as a comparator, but purpose-made comparators perform better.

As Eugene in the comments pointed out, you probably want a Schmitt trigger comparator, which, instead of a single threshold value, has two; after going high, it remains high until it reaches the lower of the two thresholds, and after going low it remains low until it reaches the higher threshold. This helps with noise immunity.