style-only: Use for instead of when loading Chainstate

Summary:
It's a bit clearer and restricts the scope of fLoaded

This is a partial backport of core#22564
https://github.com/bitcoin/bitcoin/pull/22564/commits/fe96a2e4bd87768df8001eb4117926a0977d876e

Depends on D13087

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D13088