Use `rand.Int31n()` to get power of two optimization

In cases where i+1 is a power of two, skip one modulo operation.

consul/structs/structs.go

@@ -410,7 +410,7 @@ type CheckServiceNodes []CheckServiceNode
 // Shuffle does an in-place random shuffle using the Fisher-Yates algorithm.
 func (nodes CheckServiceNodes) Shuffle() {
 	for i := len(nodes) - 1; i > 0; i-- {
-		j := rand.Int31() % int32(i+1)
+		j := rand.Int31n(int32(i + 1))
 		nodes[i], nodes[j] = nodes[j], nodes[i]
 	}
 }