stream: simplify howMuchToRead()

This slightly refactors read by moving side effects out of
howMuchToRead().

We don't actually have to set state.needReadable = true; in
howMuchToRead() since read handles 0 return as needReadable.

PR-URL: https://github.com/nodejs/node/pull/29155


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>