Summary:
_renderScene would unnecessary test if `this.state.transitionFromIndex != null` twice.
The left side and right side of the 'or' statement would always be the same.
For cleaner code I suggest to remove it.
Closes https://github.com/facebook/react-native/pull/10042
Differential Revision: D3923058
fbshipit-source-id: 7466c1f0f24eac3f9f296debd9a5e9f5320aea28