On many small farms, it was fairly common and probably still is common for them to go the hogs. The large producers have turned to composting. Many of the rendering plants have gone out of business and incinerating the carcasses is expensive and not considered environmentally responsible. There are several "recipes" to compost carcasses that provide the correct carbon to nitrogen ratio. Usually it's something like a 12" layer of litter, a 6" layer of straw, a layer of carcasses and then a 12" layer of litter and then cap it with finished compost or dirt. You can find many ag extension documents online that talk about composting mortalities.