Extract the flag selection logic out of CheckBlock

Summary: As it can be reused and/or tested when modified.

Test Plan:

make check

Reviewers: freetrader, CCulianu, #bitcoin_abc, sickpig, dagurval

Reviewed By: #bitcoin_abc, sickpig, dagurval

Subscribers: dagurval, sickpig

Differential Revision: https://reviews.bitcoinabc.org/D520