Summary:
There was a `pprTrace` in `isPredTy` that could fire under certain
scenarios, causing normal GHC users to see debugging output. This turns it into
a less chatty `WARN`, and expounds on the comment below it to add the scenario
in #13187 which triggered the `pprTrace`.

Reviewers: goldfire, austin, bgamari

Reviewed By: goldfire, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D3033

GHC Trac Issues: #13187