Make prerendering always non-blocking (#31056)

When a synchronous update suspends, and we prerender the siblings, the
prerendering should be non-blocking so that we can immediately restart
once the data arrives.

This happens automatically when there's a Suspense boundary, because we
immediately commit the boundary and then proceed to a Retry render,
which are always concurrent. When there's not a Suspense boundary, there
is no Retry, so we need to take care to switch from the synchronous work
loop to the concurrent one, to enable time slicing.