[Fizz] Unify prerender only fields to save a field (#35067)

I need to regain a field because the SuspenseBoundary type is already at
16 fields in prod, after which it deopts v8.

There are two fields that are only used in prerender to track postpones.
These are ripe to be split into an optional object so that they only
take up one field when they're not used.