Shrimp Tacos with Melon-Pineapple Salsa

1/2 small cantaloupe, about 1 1/4 lb., peeled, seeded and cut into 1/2-inch dice 

1/2 small pineapple, about 1 lb., peeled, cored and cut into 1/2-inch dice 

1/2 red onion, finely chopped 

1/2 bell pepper, seeded and finely chopped 

1 small cucumber, about 6 oz., peeled, seeded and cut into 1/2-inch dice 

1/2 jalapeño chili, seeded and finely chopped  

1/2 cup chopped fresh cilantro  

2 Tbs. fresh lime juice 

2 Tbs. olive oil 

Salt and freshly ground pepper, to taste 

24 medium shrimp, about 1 lb. total, peeled and deveined  

4 Tbs. (1/2 stick) unsalted butter 

1/2 tsp. chipotle chili powder 

1 small garlic clove, minced 

4 corn tortillas, each 6 inches in diameter