Don't visit siblings of the coroutine when looking for yields

When visiting the yields, the root is the stateNode of the coroutine. If
we treat the coroutine itself as the root, then we'll start looking
through its children.

Author: sebmarkbage

src/renderers/shared/fiber/ReactFiberCompleteWork.js

@@ -84,7 +84,8 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
   }
 
   function appendAllYields(yields : Array<mixed>, workInProgress : Fiber) {
-    let node = workInProgress.stateNode;
+    const root = workInProgress.stateNode;
+    let node = root;
     while (node) {
       if (node.tag === HostComponent || node.tag === HostText ||
           node.tag === HostPortal) {
@@ -96,11 +97,11 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
         node = node.child;
         continue;
       }
-      if (node === workInProgress) {
+      if (node === root) {
         return;
       }
       while (!node.sibling) {
-        if (!node.return || node.return === workInProgress) {
+        if (!node.return || node.return === root) {
           return;
         }
         node = node.return;