Put the corn, celery, carrot, shallots, and garlic in a large dry pot and cook over medium heat for 5 minutes, stirring occasionally. Stir in the oregano, granulated garlic, and granulated onion and cook, stirring frequently, for 5 minutes. Put the tomatoes in a blender and process on high speed until smooth. Add the tomatoes and potatoes to the pot and cook, stirring occasionally, for 5 minutes. Stir in the broth and increase the heat to medium-high. Simmer for 15 minutes. Stir in the optional epazote and simmer for 5 minutes.