Almost all garbage bags are made of polyethylene, which is thought to be the safest plastic. However, unless it is specifically "food grade" plastic, the dyes and other chemicals used in the processing of the plastic will leach into the soil.

Is it enough to really matter? I doubt it. But if you want to be on the safe side, use "food grade" garbage bags, which you can get from restaurant/kitchen supplies. They aren't expensive, but they're usually clear and somewhat flimsy. Ziploc bags meant for food storage are also obviously food grade, and made from polyethylene.

Compostable garbage bags should also be safe, but would they last a growing season? I don't know.