Add a tiny bit more tracing
Be more careful when iterating the constraint-simpifier loop (fixes Trac #8474)

We were (uselessly) iterating the simplification loop an exponential
number of times.  Lovely simple test case showed this up.

See Note [Cutting off simpl_loop] in TcSimplify
Remove canSolve in favour of canRewrite

There was no useful distinction; a simple refactoring.
Untabify and remove trailing whitespace

... plus a couple of unused variables in TcSMonad