Perform remove_returns pass before constant_propagator

The constant_propagator does not handle returns and gives unexpected
results for examples including returns. Carrying out the remove_returns
pass before contant propagation should resolve this issue.

Author: thomasspriggs

src/goto-instrument/goto_instrument_parse_options.cpp

@@ -1285,6 +1285,7 @@ void goto_instrument_parse_optionst::instrument_goto_program()
   if(cmdline.isset("constant-propagator"))
   {
     do_indirect_call_and_rtti_removal();
+    remove_returns(goto_model);
 
     log.status() << "Propagating Constants" << messaget::eom;