Make node point to builtin func they result from

This make finding the function the string results from easier.

Author: romainbrenguier

src/solvers/refinement/string_refinement_util.cpp

@@ -478,17 +478,11 @@ optionalt<exprt> string_dependencest::eval(
     return eval_string_cache[it->second];
 
   const auto node = string_nodes[it->second];
-
-  if(node.dependencies.size() == 1)
+  if(const auto &f = node.result_from)
   {
-    const auto &f = get_builtin_function(node.dependencies[0]);
-    const auto result = f.string_result();
-    if(result && *result == s)
-    {
-      const auto value_opt = f.eval(get_value);
-      eval_string_cache[it->second] = value_opt;
-      return value_opt;
-    }
+    const auto value_opt = get_builtin_function(*f).eval(get_value);
+    eval_string_cache[it->second] = value_opt;
+    return value_opt;
   }
   return {};
 }
@@ -525,7 +519,12 @@ bool add_node(
   if(
     const auto &string_result =
       dependencies.get_builtin_function(builtin_function_node).string_result())
+  {
     dependencies.add_dependency(*string_result, builtin_function_node);
+    auto &node = dependencies.get_node(*string_result);
+    node.result_from = builtin_function_node;
+
+  }
   else
     add_dependency_to_string_subexprs(
       dependencies, *fun_app, builtin_function_node, array_pool);

src/solvers/refinement/string_refinement_util.h

@@ -309,6 +309,8 @@ class string_dependencest
     std::size_t index;
     // builtin functions on which it depends
     std::vector<builtin_function_nodet> dependencies;
+    // builtin function of which it is the result
+    optionalt<builtin_function_nodet> result_from;
     // In case it depends on a builtin_function we don't support yet
     bool depends_on_unknown_builtin_function = false;