[Substitution map] When the superclass conforms to a protocol directly, use it.

When looking up a conformance in a substitution map, check whether
there is a superclass constraint that satisfies the conformance. If
so, use it directly rather than going through a slower path to find
it.

Addresses rdar://problem/46655186.

Author: DougGregor

lib/AST/SubstitutionMap.cpp

@@ -353,19 +353,17 @@ SubstitutionMap::lookupConformance(CanType type, ProtocolDecl *proto) const {
       return None;
     };
 
+  // Check whether the superclass conforms.
+  if (auto superclass = genericSig->getSuperclassBound(type)) {
+    LookUpConformanceInSignature lookup(*getGenericSignature());
+    if (auto conformance = lookup(type->getCanonicalType(), superclass, proto))
+      return conformance;
+  }
+
   // If the type doesn't conform to this protocol, the result isn't formed
   // from these requirements.
-  if (!genericSig->conformsToProtocol(type, proto)) {
-    // Check whether the superclass conforms.
-    if (auto superclass = genericSig->getSuperclassBound(type)) {
-      return LookUpConformanceInSignature(*getGenericSignature())(
-                                                 type->getCanonicalType(),
-                                                 superclass,
-                                                 proto);
-    }
-
+  if (!genericSig->conformsToProtocol(type, proto))
     return None;
-  }
 
   auto accessPath =
     genericSig->getConformanceAccessPath(type, proto);