Fix #10031 by inverting a critical test in kick_out.

Summary:
The documentation (Note [The flattening story] in TcFlatten) was
correct; it's just the implementation that was not.

Test in typecheck/should_compile/T10031

Test Plan: validate

Reviewers: austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D630

GHC Trac Issues: #10031

(cherry picked from commit cecf036f)