Move hydration code out of normal Suspense path (#24532)

* Move hydration code out of normal Suspense path

Shuffling some code around to make it easier to follow. The logic for
updating a dehydrated Suspense boundary is significantly different
from the logic for a client-rendered Suspense boundary. Most of it was
already lifted out into a separate function; this moves the remaining
hydration-specific logic out of updateSuspenseComponent and into
updateDehydratedSuspenseComponent instead.

No expected changes to program behavior.

* Extract hydration logic in complete phase, too

Same as previous step but for the complete phase. This is a separate
commit to make bisecting easier in case something breaks. The logic
is very subtle but mostly all I've done is extract it to
another function.