Combine tomatillos, chiles, 4 garlic cloves, water, and 1/2 teaspoon salt in a 4-quart saucepan and simmer, covered, stirring occasionally, until tomatillos are tender, 10 to 12 minutes. (Chiles will not be tender.)
While tomatillos are simmering, peel and devein shrimp. Rinse and pat dry, then toss with 1/2 teaspoon salt.
Working in batches if necessary, purée tomatillo mixture, oil, and cilantro in a blender until smooth (use caution when blending hot liquids).
Force through a medium-mesh sieve into a bowl, pressing hard on and then discarding solids.
Heat butter in a deep 12-inch heavy skillet over moderate heat until foam subsides, then add remaining 4 garlic cloves, 2 cups tomatillo sauce, and remaining 1/2 teaspoon salt and bring to a simmer.
Add shrimp and simmer, stirring, 2 minutes. Add remaining sauce and simmer, stirring, until shrimp are just cooked through, 2 to 3 minutes.