Avoid implicit cast int -> bool

Visual Studio warns: warning C4800: 'unsigned __int64' :
forcing value to bool 'true' or 'false' (performance warning)

Author: tautschnig

src/goto-programs/goto_model.h

@@ -92,7 +92,8 @@ class goto_modelt : public abstract_goto_modelt
 
   bool can_produce_function(const irep_idt &id) const override
   {
-    return goto_functions.function_map.count(id);
+    return goto_functions.function_map.find(id) !=
+           goto_functions.function_map.end();
   }
 };
 
@@ -127,7 +128,8 @@ class wrapper_goto_modelt : public abstract_goto_modelt
 
   bool can_produce_function(const irep_idt &id) const override
   {
-    return goto_functions.function_map.count(id);
+    return goto_functions.function_map.find(id) !=
+           goto_functions.function_map.end();
   }
 
 private: