[NFC] Add clarification comment

llvm-svn: 330677

llvm/lib/Transforms/Utils/LoopRotationUtils.cpp

@@ -259,8 +259,10 @@ bool LoopRotate::rotateLoop(Loop *L, bool SimplifiedLatch) {
     return false;
 
   // Anything ScalarEvolution may know about this loop or the PHI nodes
-  // in its header will soon be invalidated, and it can also affect its parent
-  // loops as well.
+  // in its header will soon be invalidated. We should also invalidate
+  // all outer loops because insertion and deletion of blocks that happens
+  // during the rotation may violate invariants related to backedge taken
+  // infos in them.
   if (SE)
     SE->forgetTopmostLoop(L);