[Request evaluator] Eliminate assert that fires on cycles

This assertion will trigger whenever there is a cycle with a
split-cached request. Remove the assertion.

Author: DougGregor

include/swift/AST/Evaluator.h

@@ -276,9 +276,7 @@ class Evaluator {
            typename std::enable_if<Request::hasSplitCache>::type* = nullptr>
   void cacheNonEmptyOutput(const Request &request,
                            typename Request::OutputType &&output) {
-    bool inserted = cache.insert<Request>(request, std::move(output));
-    assert(inserted && "Request result was already cached");
-    (void) inserted;
+    (void)cache.insert<Request>(request, std::move(output));
   }
 
   /// Consults the request evaluator's cache for a split-cached request.