Reset subtreeFlags in resetWorkInProgress

Alternate fix to #20942

There was already a TODO to make this change, but at the time I left it,
I couldn't think of a way that it would actually cause a bug, and I was
hesistant to change something without fully understanding the
ramifications. This was during a time when we were hunting down a
different bug, so we were especially risk averse.

What I should have done in retrospect is put the change behind a flag
and tried rolling it out once the other bug had been flushed out.

OTOH, now we have a regression test, which wouldn't have otherwise, and
the bug it caused rarely fired in production.

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

@@ -388,10 +388,7 @@ export function resetWorkInProgress(workInProgress: Fiber, renderLanes: Lanes) {
     workInProgress.lanes = current.lanes;
 
     workInProgress.child = current.child;
-    // TODO: `subtreeFlags` should be reset to NoFlags, like we do in
-    // `createWorkInProgress`. Nothing reads this until the complete phase,
-    // currently, but it might in the future, and we should be consistent.
-    workInProgress.subtreeFlags = current.subtreeFlags;
+    workInProgress.subtreeFlags = NoFlags;
     workInProgress.deletions = null;
     workInProgress.memoizedProps = current.memoizedProps;
     workInProgress.memoizedState = current.memoizedState;

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

@@ -388,10 +388,7 @@ export function resetWorkInProgress(workInProgress: Fiber, renderLanes: Lanes) {
     workInProgress.lanes = current.lanes;
 
     workInProgress.child = current.child;
-    // TODO: `subtreeFlags` should be reset to NoFlags, like we do in
-    // `createWorkInProgress`. Nothing reads this until the complete phase,
-    // currently, but it might in the future, and we should be consistent.
-    workInProgress.subtreeFlags = current.subtreeFlags;
+    workInProgress.subtreeFlags = NoFlags;
     workInProgress.deletions = null;
     workInProgress.memoizedProps = current.memoizedProps;
     workInProgress.memoizedState = current.memoizedState;