ConnectBlock : count sigops in one place

Summary:
We can merge the coinbase sigops special case into the second
loop in such a manner. It works because GetTransactionSigOpCount
functions properly on coinbase txns.

The one wrinkle is that normal transactions need CheckTxInputs
to be run on them, but CheckTxInputs does not permit coinbases.

(Alternatively we could modify CheckTxInputs to allow coinbases,
and have it set fee=0. But I think I prefer this way.)

Depends on D5076

Test Plan: ninja check-extended

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5077