Introduce DeepRejectCtxt::substs_refs_may_unify.

It factors out a repeated code pattern.

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -2542,8 +2542,10 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
         // substitution if we find that any of the input types, when
         // simplified, do not match.
         let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::ForLookup };
-        iter::zip(obligation.predicate.skip_binder().trait_ref.substs, impl_trait_ref.substs)
-            .any(|(obl, imp)| !drcx.generic_args_may_unify(obl, imp))
+        !drcx.substs_refs_may_unify(
+            obligation.predicate.skip_binder().trait_ref.substs,
+            impl_trait_ref.substs,
+        )
     }
 
     /// Normalize `where_clause_trait_ref` and try to match it against