DataFlow: Permit local flow between post-update nodes

Author: asgerf

shared/dataflow/codeql/dataflow/internal/DataFlowImplConsistency.qll

@@ -254,7 +254,10 @@ module MakeConsistency<
 
   query predicate postWithInFlow(PostUpdateNode n, string msg) {
     not clearsContent(n, _) and
-    simpleLocalFlowStep(_, n, _) and
+    exists(Node pred |
+      simpleLocalFlowStep(pred, n, _) and
+      not pred instanceof PostUpdateNode
+    ) and
     not Input::postWithInFlowExclude(n) and
     msg = "PostUpdateNode should not be the target of local flow."
   }