Handle the likely:True case in CmmContFlowOpt
It's better to fall through to the likely case than to jump to it.
We optimize for this in CmmContFlowOpt when likely:False.
This commit extends the logic there to handle cases with likely:True
Test Plan: ci
Reviewers: bgamari, simonmar
Reviewed By: bgamari
Subscribers: simonmar, alexbiehl, rwbarton, thomie, carter
Differential Revision: https://phabricator.haskell.org/D4306