Constant-time compute a goto_programt::targett from const_targett

This enables use of const_targett is almost all cases, with actual modification
using this on-demand conversion.

Author: tautschnig

src/goto-programs/goto_program_template.h

@@ -248,6 +248,19 @@ class goto_program_templatet
   //! The list of instructions in the goto program
   instructionst instructions;
 
+  // Convert a const_targett to a targett - use with care and avoid
+  // whenever possible
+  targett const_cast_target(const_targett t)
+  {
+    return instructions.erase(t, t);
+  }
+
+  // Dummy for templates with possible const contexts
+  const_targett const_cast_target(const_targett t) const
+  {
+    return t;
+  }
+
   void get_successors(
     targett target,
     targetst &successors);