This for some reason or the other and makes it into the final
binary. I've added the check to ContFlowOpt as that seems
like a logical place for this.

In a regular nofib run there were 30 occurences of this pattern.

Test Plan: ci

Reviewers: bgamari, simonmar, dfeuer, jrtc27, tdammers

Reviewed By: bgamari, simonmar

Subscribers: tdammers, dfeuer, rwbarton, thomie, carter

GHC Trac Issues: #15188

Differential Revision: https://phabricator.haskell.org/D4740