Fix iterator-invalidation issue

Inserting a new key into a DenseMap potentially invalidates iterators into that
map. Trying to fix an issue from r289755 triggering this assertion:

  Assertion `isHandleInSync() && "invalid iterator access!"' failed.

llvm-svn: 289757

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -3529,7 +3529,9 @@ void ScalarEvolution::addAffectedFromOperands(const SCEV *S) {
       if (AMI == AffectedMap.end())
         continue;
 
-      AffectedMap[S].insert(AMI->second.begin(), AMI->second.end());
+      auto &ISet = AffectedMap[S];
+      AMI = AffectedMap.find(Op);
+      ISet.insert(AMI->second.begin(), AMI->second.end());
     }
 }