refactor: Extract ParseOpCode from ParseScript
Summary:
Seems more natural to have mapOpNames "hidden" in ParseOpCode than in ParseScript.
A second lookup in mapOpNames is also removed.
This is a backport of core#19851
Test Plan: ninja all check-all
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D10812