Roast the unpeeled garlic in an ungreased small skillet over medium heat, turning from time to time, until completely soft and blackened in places, about 15 minutes; cool and peel. In a blender or food processor, combine the garlic, tomatoes and chipotle chiles; blend to a smooth puree. In a medium (10-inch) skillet, heat the oil over medium-high. Add the puree and stir until reduced to the consistency of tomato paste, 5 minutes or so. Stir in the cream, pepper and oregano, reduce the heat to medium and let simmer for several minutes for the flavors to come together. Taste and season with salt, usually 1/2 teaspoon. Add the shrimp and stir until they are just cooked through (just losing their translucency at the center), about 3 minutes. Serve immediately with warm tortillas.