RequirementMachine: Tighten verifyRewriteRules() further

We should never have a rule with a protocol symbol on the *right* hand
side, like (T => U.[P]).

Author: slavapestov

lib/AST/RequirementMachine/RewriteSystem.cpp

@@ -591,10 +591,7 @@ void RewriteSystem::verifyRewriteRules(ValidityPolicy policy) const {
         ASSERT_RULE(symbol.getKind() != Symbol::Kind::GenericParam);
       }
 
-      // Completion can produce rules like [P:T].[Q:R] => [P:T].[Q]
-      // which are immediately simplified away.
-      if (!rule.isRHSSimplified() &&
-          index != 0) {
+      if (index != 0) {
         ASSERT_RULE(symbol.getKind() != Symbol::Kind::Protocol);
       }
     }