Add Placement bit without removing others (facebook#20398)

When scheduling a Placement effect, we should add the Placement bit
without resetting the others.

In the old fork, there are no flags to reset, anyway, since by the
time we reach the child reconciler, the flags will have already been
reset.

However, in the effects refactor, "static" flags are not reset, so this
can actually manifest as a bug. See facebook#20285 for a regression test.

packages/react-reconciler/src/ReactChildFiber.new.js

@@ -342,15 +342,15 @@ function ChildReconciler(shouldTrackSideEffects) {
       const oldIndex = current.index;
       if (oldIndex < lastPlacedIndex) {
         // This is a move.
-        newFiber.flags = Placement;
+        newFiber.flags |= Placement;
         return lastPlacedIndex;
       } else {
         // This item can stay in place.
         return oldIndex;
       }
     } else {
       // This is an insertion.
-      newFiber.flags = Placement;
+      newFiber.flags |= Placement;
       return lastPlacedIndex;
     }
   }
@@ -359,7 +359,7 @@ function ChildReconciler(shouldTrackSideEffects) {
     // This is simpler for the single child case. We only need to do a
     // placement for inserting new children.
     if (shouldTrackSideEffects && newFiber.alternate === null) {
-      newFiber.flags = Placement;
+      newFiber.flags |= Placement;
     }
     return newFiber;
   }

packages/react-reconciler/src/ReactChildFiber.old.js

@@ -342,15 +342,15 @@ function ChildReconciler(shouldTrackSideEffects) {
       const oldIndex = current.index;
       if (oldIndex < lastPlacedIndex) {
         // This is a move.
-        newFiber.flags = Placement;
+        newFiber.flags |= Placement;
         return lastPlacedIndex;
       } else {
         // This item can stay in place.
         return oldIndex;
       }
     } else {
       // This is an insertion.
-      newFiber.flags = Placement;
+      newFiber.flags |= Placement;
       return lastPlacedIndex;
     }
   }
@@ -359,7 +359,7 @@ function ChildReconciler(shouldTrackSideEffects) {
     // This is simpler for the single child case. We only need to do a
     // placement for inserting new children.
     if (shouldTrackSideEffects && newFiber.alternate === null) {
-      newFiber.flags = Placement;
+      newFiber.flags |= Placement;
     }
     return newFiber;
   }