The Catechism of the Catholic Church, as you may know, says this about torture:
Torture which uses physical or moral violence to extract confessions, punish the guilty, frighten opponents, or satisfy hatred is contrary to respect for the person and for human dignity. [CCC 2297]
Recognizing that what is "contrary to respect for the person and for human dignity" is never morally licit, and that the Catechism is a sure norm for teaching the Catholic faith, how can Catholics conclude that torturing a prisoner to obtain life-saving information could ever be licit?
Clearly, the argument has to be that the kind of torture condemned in the Catechism is not the kind of torture they conclude could be licit.
The Catechism itself provides a suggestion for how that might be done. We can understand the various purposes for torture listed as different motives that specify different moral objects. That is, we could say that "torture" is a general human act of using physical or moral violence, and "confession extracting torture," "guilty punishing torture," "opponent frightening torture," and "hatred satisfying torture" are four species of torture, all of which are evil in their object and therefore everywhere and always immoral.
Thus, we might posit a fifth species of torture -- say, "information obtaining torture." We could establish that this kind of torture is not evil in its object if we can prove both of the following:
"Information obtaining torture" is a species of torture distinct from the four explicitly condemned in the Catechism.
"Information obtaining torture" can be compatible with respect for the person and for human dignity.
This is, by the way, the approach Fr. Brian Harrison proposes in somewhat different form (here and here), a fact I mention since Fr. Harrison's proposal is widely referred to among pro-torture Catholics.
Neither of these assertions is true, so torturing a prisoner to obtain life-saving information can never be licit. Granted, we have to look beyond this one statement of the Catechism to see that they're both false (although just one being false would suffice).
And yes, we do have to look, because as the Catechism goes on to say of "cruel practices ... used by legitimate governments to maintain law and order":