in what way is it analog?
in my opinion it is either "on" (light is reflected into the receiver) or "off" (no or not enough light falls into the receiver)...
so u can use any pin that u configured with these commands:
pinMode(inputPin, INPUT); digitalWrite(inputPin, HIGH);
or do u want to know if "alot" or "just little" light falls into the receiver?
then u might use an analog pin...
but i dont c how that is necessary to build a tachometer...
i think u just need to count the "light on" phases per minute...
or the time between two "light on" phases...