[PM] Don't walk the AM's ResultsList if nothing was invalidated.

[jlebar]

Summary

Previously in AnalysisManager::invalidate(), we would walk the full
ResultsList even if we knew that nothing was invalidated.

llvm/trunk/include/llvm/IR/PassManager.h

for (auto &AnalysisResultPair : ResultsList) {
           IsResultInvalidated.insert({ID, Result.invalidate(IR, PA, Inv)})
               .second;
       (void)Inserted;
       assert(Inserted && "Should never have already inserted this ID, likely "
                          "indicates a cycle!");
     }
 
     // Now erase the results that were marked above as invalidated.
+    if (!IsResultInvalidated.empty()) {
       for (auto I = ResultsList.begin(), E = ResultsList.end(); I != E;) {
         AnalysisKey *ID = I->first;
         if (!IsResultInvalidated.lookup(ID)) {
           ++I;
           continue;
         }
 
         if (DebugLogging)
-        dbgs() << "Invalidating analysis: " << this->lookupPass(ID).name()
+          dbgs() << "Invalidating analysis: " << this->lookUpPass(ID).name()
                  << "\n";
 
         I = ResultsList.erase(I);
         AnalysisResults.erase({ID, &IR});
       }
+    }
+
     if (ResultsList.empty())
       AnalysisResultLists.erase(&IR);
-
-    return;
   }
 
 private:
   /// \brief Lookup a registered analysis pass.
   PassConceptT &lookupPass(AnalysisKey *ID) {
     typename AnalysisPassMapT::iterator PI = AnalysisPasses.find(ID);
     assert(PI != AnalysisPasses.end() &&
            "Analysis passes must be registered prior to being queried!");