Warn on unreachable instances in chains

[rhendric]

Summary

As suggested here, it would be nice to detect when an instance chain contains instances that could never be selected because they're dominated by preceding instances in the chain and warn the user. Perhaps also include a message mentioning that constraints aren't used until after an instance is selected, or suggesting a reordering of the instances.

Motivation

See #4440 for this confusing someone in the wild.

Proposal

TBD, but doesn't seem that complicated.

Examples

Case where we might mention that constraints aren't used until after selection:

instance Show a => Foo a
else instance Foo a

Case where we might suggest a reordering:

instance Foo String a
else instance Foo a String
else instance Foo String String