Refactor to reduce duplicated code

apollo-ios-codegen/Sources/IR/GraphQLCompositeType+Identifiable.swift

@@ -0,0 +1,16 @@
+import GraphQLCompiler
+
+extension GraphQLCompositeType {
+  var isIdentifiable: Bool {
+    switch(self) {
+    case let interface as GraphQLInterfaceType:
+      return interface.keyFields == ["id"]
+
+    case let object as GraphQLObjectType:
+      return object.keyFields == ["id"]
+
+    default:
+      return false
+    }
+  }
+}

apollo-ios-codegen/Sources/IR/IR+ComputedSelectionSet.swift

@@ -25,17 +25,8 @@ public struct ComputedSelectionSet {
     guard direct?.fields["id"] != nil || merged.fields["id"] != nil else {
       return false
     }
-    if let type = typeInfo.parentType as? GraphQLObjectType,
-       type.keyFields == ["id"] {
-      return true
-    }
-
-    if let type = typeInfo.parentType as? GraphQLInterfaceType,
-       type.keyFields == ["id"] {
-      return true
-    }
-
-    return false
+
+    return typeInfo.parentType.isIdentifiable
   }
 
   // MARK: Dynamic Member Subscript

apollo-ios-codegen/Sources/IR/IR+NamedFragment.swift

@@ -30,14 +30,7 @@ public class NamedFragment: Definition, Hashable, CustomDebugStringConvertible {
       return field.name == "id"
     }) else { return false }
 
-    switch(type) {
-    case let interface as GraphQLInterfaceType:
-      return interface.keyFields == ["id"]
-    case let object as GraphQLObjectType:
-      return object.keyFields == ["id"]
-    default:
-      return false
-    }
+    return type.isIdentifiable
   }
 
   init(