Make simplify_exprt::simplify_node virtual

While simplify is already virtual, it isn't really a useful extension
point because it's called only at the start of the simplification, and
all the real work is handed off to simplify_rec which, in turn, calls
simplify_node, and then both of which can be recursed into from further
down the expression tree.

In order to customise the expression simplification we need to be able
to hook into that recursive call, and simplify_node is the most useful
place to do that.

Author: jezhiggins

src/util/simplify_expr_class.h

@@ -233,7 +233,7 @@ class simplify_exprt
   simplify_inequality_pointer_object(const binary_relation_exprt &);
 
   // main recursion
-  NODISCARD resultt<> simplify_node(exprt);
+  NODISCARD virtual resultt<> simplify_node(exprt);
   bool simplify_node_preorder(exprt &expr);
   NODISCARD resultt<> simplify_rec(const exprt &);