RequirementMachine: Relax assertion in verifyRewriteSystem()

We can end up with a rule that has a protocol symbol on the right hand side:

    X.Y.Z => X.W.[P]

This will occur if X.W.[P] was obtained by simplifying a term X.W.U.V via
a rule (U.V => [P]), and before completion discovers a rule
(X.W.[P] => X.W).

Fixes rdar://problem/94854326, rdar://problem/94980084.

lib/AST/RequirementMachine/RewriteSystem.cpp

@@ -591,7 +591,7 @@ void RewriteSystem::verifyRewriteRules(ValidityPolicy policy) const {
         ASSERT_RULE(symbol.getKind() != Symbol::Kind::GenericParam);
       }
 
-      if (index != 0) {
+      if (index != 0 && !rule.isRHSSimplified()) {
         ASSERT_RULE(symbol.getKind() != Symbol::Kind::Protocol);
       }
     }