Pass flags down to GetSigOpCount

Summary: This will ensure we can use flags to tweak the way sigops are counted in the future. This is necessary for the activation of OP_CHECKDATASIG.

Test Plan: Added test cases to ensure behavior changes as expected when the P2SH flags is flipped.

Reviewers: #bitcoin_abc, jasonbcox, schancel

Reviewed By: #bitcoin_abc, jasonbcox, schancel

Subscribers: schancel, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1601