Consider this example:
protocol P {
associatedtype X : P where X == Self
}
Clearly the conformance requirement 'X : P' is redundant, but previously
nothing in our formulation would make it so.
Here is the rewrite system:
(1) [P:X].[P] => [P:X]
(2) [P:X] => [P]
These two terms overlap on [P:X].[P]; resolving the critical pair introduces
the 'identity conformance' [P].[P] => [P]. Homotopy reduction would delete
this conformance, but at this point, [P:X].[P] => [P:X] would no longer be
redundant, since nothing else proves that [P].[P] => [P].
Now that [P].[P] => [P] is a permanent rule, we can handle this properly;
any conformance that appears in a rewrite loop together with an identity
conformance without context is completely redundant; it is equivalent to the
empty generating conformance path.
