This patch fixes the redundant superclass expansion
in Trac #14774.

The main change is to fix TcInterac.solveOneFromTheOther, so
that it does not prefer a work-item with a binding if that binding
transitively depends on the inert item we are comparing it with.

Explained in Note [Replacement vs keeping] in TcInert, esp
item (c) of the "Constraints coming from the same level" part.

To make this work I refactored out a new function
TcEvidence.findNeededEvVars, which was previously buried
inside TcSimplify.neededEvVars.

I added quite a few more comments and signposts about superclass
expansion.