Allow more hooks to be added when replaying mount

Currently, if you call setState in render, you must render the exact
same hooks as during the first render pass.

I'm about to add a behavior where if something suspends, we can reuse
the hooks from the previous attempt. That means during initial render,
if something suspends, we should be able to reuse the hooks that were
already created and continue adding more after that. This will error
in the current implementation because of the expectation that every
render produces the same list of hooks.

In this commit, I've changed the logic to allow more hooks to be added
when replaying. But only during a mount — if there's already a current
fiber, then the logic is unchanged, because we shouldn't add any
additional hooks that aren't in the current fiber's list. Mounts are
special because there's no current fiber to compare to.

I haven't change any other behavior yet. The reason I've put this into
its own step is there are a couple tests that intentionally break the
Hook rule, to assert that React errors in these cases, and those happen
to be coupled to the behavior. This is undefined behavior that is always
accompanied by a warning and/or error. So the change should be safe.