Fix recomputation of TypeRep in the instance for Typeable (s a) (#9203)

Summary:
Every time we called typeRep on a type application, we were
recomputing the TypeRep.  This showed up in a benchmark I had: #9203.

Test Plan: Benchmark from #9203.

Reviewers: simonpj, austin

Subscribers: simonmar, relrod

Differential Revision: https://phabricator.haskell.org/D19