C++: Remove noise from argHasPostUpdate check

This consistency check seems to have value for AST data flow, but I've
disabled it on the IR for now.

This commit also includes two unrelated changes that seem to fix a
semantic merge conflict.

cpp/ql/src/semmle/code/cpp/dataflow/internal/DataFlowPrivate.qll

@@ -301,4 +301,19 @@ int accessPathLimit() { result = 5 }
  *
  * This predicate is only used for consistency checks.
  */
-predicate isImmutableOrUnobservable(Node n) { none() }
+predicate isImmutableOrUnobservable(Node n) {
+  // Is the null pointer (or something that's not really a pointer)
+  exists(n.asExpr().getValue())
+  or
+  // Isn't a pointer or is a pointer to const
+  forall(DerivedType dt | dt = n.asExpr().getActualType() |
+    dt.getBaseType().isConst()
+    or
+    dt.getBaseType() instanceof RoutineType
+  )
+  or
+  // Isn't something we can track
+  n.asExpr() instanceof Call
+  // The above list of cases isn't exhaustive, but it narrows down the
+  // consistency alerts enough that most of them are interesting.
+}

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

@@ -210,4 +210,8 @@ int accessPathLimit() { result = 5 }
  *
  * This predicate is only used for consistency checks.
  */
-predicate isImmutableOrUnobservable(Node n) { none() }
+predicate isImmutableOrUnobservable(Node n) {
+  // The rules for whether an IR argument gets a post-update node are too
+  // complex to model here.
+  any()
+}