-baby laxative is indeed used to cut cocaine with, but:
How widespread it is used as such IDK, and don't really think anyone could give an even slightly approximate answer to that question.
Also, even if the coke is cut with baby laxative in the 50-60% range, and thus over half of every line/dose is pure baby laxative, I don't think that even that amount would result in a full grown adult having uncontrollable bowl movements.
Most likely, and IME and IMO, the need to go to the bathroom is due to cocaine's stimulant properties. An analogy or really just another example of this phenomena would be how coffee, cigarettes, and Rx stimulants like adderall (sic?), other various types of amphetamines, and ritalin.
All of those drugs are stimulants, (yes, while mild, the nicotine in cigarettes is a stimulant) and produce the same need to use the bathroom that cocaine does.