With the planned lazy TAIT system, this will not really make sense anymore anyway.

Previously each opaque type instantiation would create new inference vars, even for the same opaque type/substs combination. Now there is a global map in InferCtxt that gets filled whenever we encounter an opaque type.

This allows opaque type inference to check for defining uses without having to pass down that def id via function arguments to every method that could possibly cause an opaque type to be compared with a concrete type

…lures