The part of the corn kernel that is visible in human feces is the outer coat of bran called the pericarp (see this image). This is a common structure in cereal grains like corn and wheat. However, the bulk of the corn kernal is starchy endosperm, which is readily digestible by humans and provides carbohydrates and vitamins A and C. The non-digestible (by humans) pericarp does help to keep fecal material soft.
Plastic bags do not provide those same benefits. Studies on human consumption of plastic bags are lacking, for obvious reasons. However, marine organisms like sea turtles unfortunately consume large amounts of plastic debris (Derraik 2002). Bugoni et al. (2001) found that plastic debris caused mortality for 13% of the green turtles in their study. A review of 408 leatherback sea turtle autopsy records by Mrosovsky et al. (2009) found plastic inside 34% of turtles. The mortality in the sea turtles may be due to the accumulation of plastics in the gut which can block movement of food, alter feeding behaviors, and prevent absorbtion of nutrients.
If humans were to consume chopped up plastic bags, the evidence suggests that instead of receiving the same benefits as we do from whole kernal corn we are much more likely to be harmed. Eat your corn, recycle your plastic.
Bugoni, L. et al., 2001. Marine debris and human impacts on sea turtles in southern Brazil. Marine Pollution Bulletin 42: 1330-1334.
Derraik, J.G.B. 2002. The pollution of the marine environment by plastic debris: A review. Marine Pollution Bulletin 44: 842-852.
Mrosovsky, N. et al. 2009. Leatherback turtles: The menance of plastic. Marine Pollution Bulletin 58: 287-289.