Bug: Layout effects don't re-fire in 18 on Suspense re-showing

[gaearon]

Intended behavior: reactwg/react-18#31

Sandbox: https://codesandbox.io/s/react-18-layouteffect-semantics-forked-m02w5

Expected: when suspended content is hidden and then shown, we get layout cleanup followed by layout setup.
Actual: only layout cleanup happens.

[bvaughn]

I think this is a subtreeFlags issue. In commit work, where we check the Offscreen tree fiber.subtreeFlags & LayoutMask, the test you added shows no layout flags so we skip the subtree.

We intentionally removed the subtreeFlags checks for most places, since the values aren't 100% reliable and the check is just an optimization, but it looks like @acdlite added this one in #21386.