I created a profile of GHC compiling a large commercial project and noticed that `matchInstEnv` accounted for 50% of time spent typechecking the project. That seems like rather a lot. Zooming into just `matchInstEnv`..  It seems that the cost is split quite evenly between `tc_unify_tys_fg`, `tc_match_tys` and `$instIsVisible`, `instIsVisible` looks quite cheap so I wondered if this function is getting hammered an ungodly number of times.