Hole-y partial type signatures greatly slow down compile times

This time, I actually mean it. :)

Originally reported here, I distilled the example from this comment into a one file test case. Sigs.hs is exactly like NoSigs.hs, except for the fact that it has a bunch of extra type signatures that have a lot of holes. On my machine, this is what compilation times are (I gave up timing after 15 minutes):

GHC version	8.0.2	8.2.1	8.4.1 (445554b6)
`NoSigs.hs`	24.13s	22.93s	34.05s
`Sigs.hs`	\>15m	\~13m	\>15m