Move subclass inheritance to end (#7772)

We were using `Object.create` to setup the prototype chain at the start of the class definition, which lead to #7771.

I was a bit worried about a speed hit, but it seems everyone optimizes the two patterns the same way.
https://jsbench.github.io/#f9fca52407643d96458a35763b201215

Fixes #7771.

packages/babel-plugin-transform-react-jsx/test/fixtures/react/optimisation.react.constant-elements/output.js

@@ -11,8 +11,6 @@ let App =
 function (_React$Component) {
   "use strict";
 
-  babelHelpers.inherits(App, _React$Component);
-
   function App() {
     babelHelpers.classCallCheck(this, App);
     return babelHelpers.possibleConstructorReturn(this, babelHelpers.getPrototypeOf(App).apply(this, arguments));
@@ -31,5 +29,6 @@ function (_React$Component) {
     </div>;
     }
   }]);
+  babelHelpers.inherits(App, _React$Component);
   return App;
 }(React.Component);