Move output eligibility to a separate function

Summary: This is a partial backport of Core PR10637 : https://github.com/bitcoin/bitcoin/pull/10637/commits/ce7435cf1ef36109595be9a3a3955afdff1d63e4

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3245