C++: Hide dataflow nodes if they're just used for flow-through for read steps or store steps.

cpp/ql/lib/semmle/code/cpp/ir/dataflow/internal/DataFlowPrivate.qll

@@ -299,7 +299,17 @@ predicate isImmutableOrUnobservable(Node n) {
 }
 
 /** Holds if `n` should be hidden from path explanations. */
-predicate nodeIsHidden(Node n) { n instanceof OperandNode and not n instanceof ArgumentNode }
+predicate nodeIsHidden(Node n) {
+  n instanceof OperandNode and not n instanceof ArgumentNode
+  or
+  StoreNodeFlow::flowThrough(n, _) and
+  not StoreNodeFlow::flowOutOf(n, _) and
+  not StoreNodeFlow::flowInto(_, n)
+  or
+  ReadNodeFlow::flowThrough(n, _) and
+  not ReadNodeFlow::flowOutOf(n, _) and
+  not ReadNodeFlow::flowInto(_, n)
+}
 
 class LambdaCallKind = Unit;
 

cpp/ql/lib/semmle/code/cpp/ir/dataflow/internal/DataFlowUtil.qll

@@ -840,7 +840,10 @@ private predicate adjacentDefUseFlow(Node nodeFrom, Node nodeTo) {
   )
 }
 
-private module ReadNodeFlow {
+/**
+ * INTERNAL: Do not use.
+ */
+module ReadNodeFlow {
   /** Holds if the read node `nodeTo` should receive flow from `nodeFrom`. */
   predicate flowInto(Node nodeFrom, ReadNode nodeTo) {
     nodeTo.isInitial() and