Fallback to current props if memoizedProps is null

If work has progressed on a state update that gets resumed
because of another state up, then we won't have an new
pendingProps, and we also won't have any memoizedProps because
it got aborted before completing. In that case, we can just
fallback to the current props.

I think that they can't have diverged because the only way they
diverge is if there is new props.

This lets us bail out on state only updates in more cases which
the unit tests reflect.

src/renderers/shared/fiber/ReactFiberClassComponent.js

@@ -165,12 +165,12 @@ module.exports = function(scheduleUpdate : (fiber: Fiber, priorityLevel : Priori
   function updateClassInstance(current : Fiber, workInProgress : Fiber) : boolean {
     const instance = workInProgress.stateNode;
 
-    const oldProps = current.memoizedProps;
+    const oldProps = workInProgress.memoizedProps || current.memoizedProps;
     let newProps = workInProgress.pendingProps;
     if (!newProps) {
       // If there aren't any new props, then we'll reuse the memoized props.
       // This could be from already completed work.
-      newProps = workInProgress.memoizedProps;
+      newProps = oldProps;
       if (!newProps) {
         throw new Error('There should always be pending or memoized props.');
       }
@@ -199,7 +199,7 @@ module.exports = function(scheduleUpdate : (fiber: Fiber, priorityLevel : Priori
 
     if (typeof instance.shouldComponentUpdate === 'function' &&
         !(updateQueue && updateQueue.isForced) &&
-        workInProgress.memoizedProps !== null &&
+        oldProps !== null &&
         !instance.shouldComponentUpdate(newProps, newState)) {
       // TODO: Should this get the new props/state updated regardless?
       return false;

src/renderers/shared/fiber/__tests__/ReactIncremental-test.js

@@ -625,10 +625,10 @@ describe('ReactIncremental', () => {
     // Normally shouldComponentUpdate->false is not enough to determine that we
     // can safely reuse the old props, but I think in this case it would be ok,
     // since it is a resume of already started work.
-    // Because of the above we can also not reuse the work of Bar because the
+    // Because of the above we can not reuse the work of Bar because the
     // rerender of Content will generate a new element which will mean we don't
     // auto-bail out from Bar.
-    expect(ops).toEqual(['Content', 'Bar', 'Middle']);
+    expect(ops).toEqual(['Bar', 'Middle']);
 
   });
 

src/renderers/shared/fiber/__tests__/ReactIncrementalSideEffects-test.js

@@ -598,7 +598,7 @@ describe('ReactIncrementalSideEffects', () => {
       ),
     ]);
 
-    expect(ops).toEqual(['Baz', 'Bar', 'Baz', 'Bar', 'Bar']);
+    expect(ops).toEqual(['Bar', 'Baz', 'Bar', 'Bar']);
   });
 
   it('deprioritizes setStates that happens within a deprioritized tree', () => {