Do not add a 'solved dict' for quantified constraints
GHC has a wonderful-but-delicate mechanism for building recursive dictionaries by adding a goal to the "solved dictionaries" before solving the sub-goals. See Note [Solved dictionaries] in TcSMonad
Ticket #17267 (closed) showed that if you use this mechanism for local /quantified/ constraints you can get a loop -- or even unsafe coerce. This patch fixes the bug.
Make TcSMonad.addSolvedDict be conditional on using a /top level/ instance, not a quantified one.
Moreover, we /also/ don't want to add a solved dict for equalities (a~b).
Add lots more comments to Note [Solved dictionaries] to explain the above cryptic stuff.
Extend InstanceWhat to identify those strange built-in equality instances.
A couple of other things along the way
Delete the unused Type.isIPPred_maybe.
Stop making addSolvedDict conditional on not being an impolicit parameter. This comes from way back. But it's irrelevant now because IP dicts are never solved via an instance.