Firstly, strictly speaking, you can't cook potatoes (or any other food product) for your pigs in your own kitchen. The rules on feeding products which have passed through a kitchen, domestic or commercial, are very clear.
Having cleared that one up, I "believe" that some people cook potatoes and other veg in a separate brazier outside specifically for their pigs. I will leave you to ponder that one....
Anyway, back to potatoes. Most pigs are a bit fussy when it comes to them and won't eat them raw unless they are really desperate (and they shouldn't get to that stage). As a general rule, pigs don't eat what won't agree with them, and potatoes in large quantities certainly don't. They don't digest starch very well - although cooking potatoes does break down the starch to a more acceptable level.
In any case, potatoes aren't a good source of nutrition. Okay, so they might fill the pig up in the same way as barley or bran, but they won't contribute much to overall nutrition.
Please don't feed green or sprouting potatoes. They contain the toxin solanine which can cause stomach upsets - and some pig breeders I know have had pigs killed as a result of eating large amounts. Fortunately, green potatoes taste bitter, so pigs generally have the good sense to avoid them. However, if hungry, pigs will eat anything, so be warned.
Personally, I wouldn't take the chance of feeding haulms/tubers.