pretty-print expression instructions

This adds pretty-printing for goto-program expression instructions.

Author: kroening

regression/goto-instrument/remove-calls-no-body2/test.desc

@@ -6,8 +6,8 @@ main.c
 ^VERIFICATION SUCCESSFUL$
 func3\(\)
 func4\(\)
-567\)
-285\)
+567$
+285$
 ret1 :=.*nondet.*
 --
 func1\(.*\)

src/goto-programs/goto_program.cpp

@@ -124,6 +124,11 @@ std::ostream &goto_programt::instructiont::output(std::ostream &out) const
             << '\n';
         break;
       }
+      else if(code.get_statement() == ID_expression)
+      {
+        out << "EXPRESSION " << format(code.op0()) << '\n';
+        break;
+      }
       else if(code.get_statement() == ID_havoc_object)
       {
         out << "HAVOC_OBJECT " << format(code.op0()) << '\n';