Summary:
This is a first step for #9988

It turns out that bringing m_fun_id_infix through the renamer is
actually very simple, affecting the internals of rnMatch' only.

Is this simple enough to hit 7.10.1?

Test Plan: ./validate

Reviewers: hvr, simonpj, austin

Reviewed By: simonpj, austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D639

GHC Trac Issues: #9988