Note [Deterministic simplifyInstanceContexts]

Canonicalisation uses nonDetCmpType which is nondeterministic. Sorting
with nonDetCmpType puts the returned lists in a nondeterministic order.
If we were to return them, we'd get class constraints in
nondeterministic order.

Consider:

  data ADT a b = Z a b deriving Eq

The generated code could be either:

  instance (Eq a, Eq b) => Eq (Z a b) where

Or:

  instance (Eq b, Eq a) => Eq (Z a b) where

To prevent the order from being nondeterministic we only
canonicalize when comparing and return them in the same order as
simplifyDeriv returned them.
See also Note [nonDetCmpType nondeterminism]