[SILGen] Remove a vestigial use of getSelfTypeInContext().

Author: DougGregor

lib/SILGen/SILGenApply.cpp

@@ -86,13 +86,6 @@ replaceSelfTypeForDynamicLookup(ASTContext &ctx,
                               ctx);
 }
 
-static Type getExistentialArchetype(SILValue existential) {
-  CanType ty = existential->getType().getSwiftRValueType();
-  if (ty->is<ArchetypeType>())
-    return ty;
-  return cast<ProtocolType>(ty)->getDecl()->getSelfTypeInContext();
-}
-
 /// Retrieve the type to use for a method found via dynamic lookup.
 static CanSILFunctionType getDynamicMethodLoweredType(SILGenFunction &gen,
                                            SILValue proto,
@@ -103,7 +96,8 @@ static CanSILFunctionType getDynamicMethodLoweredType(SILGenFunction &gen,
   // Determine the opaque 'self' parameter type.
   CanType selfTy;
   if (methodName.getDecl()->isInstanceMember()) {
-    selfTy = getExistentialArchetype(proto)->getCanonicalType();
+    selfTy = proto->getType().getSwiftRValueType();
+    assert(selfTy->is<ArchetypeType>() && "Dynamic lookup needs an archetype");
   } else {
     selfTy = proto->getType().getSwiftType();
   }