const and protect self_loops_to_assumptions

This goto_symext member is never written to and serves merely as a cache
for a front-end option, so this commit makes it const and protected.

Author: karkhaz

src/cbmc/bmc.h

@@ -88,8 +88,6 @@ class bmct:public safety_checkert
   {
     symex.record_coverage=
       !options.get_option("symex-coverage-report").empty();
-    symex.self_loops_to_assumptions =
-      options.get_bool_option("self-loops-to-assumptions");
   }
 
   virtual resultt run(const goto_functionst &goto_functions)

src/goto-symex/goto_symex.h

@@ -60,9 +60,10 @@ class goto_symext
       doing_path_exploration(options.is_set("paths")),
       allow_pointer_unsoundness(
         options.get_bool_option("allow-pointer-unsoundness")),
-      self_loops_to_assumptions(true),
       language_mode(),
       constant_propagation(options.get_bool_option("propagation")),
+      self_loops_to_assumptions(
+        options.get_bool_option("self-loops-to-assumptions")),
       outer_symbol_table(outer_symbol_table),
       ns(outer_symbol_table),
       target(_target),
@@ -210,14 +211,13 @@ class goto_symext
   // these bypass the target maps
   virtual void symex_step_goto(statet &, bool taken);
 
-  bool self_loops_to_assumptions;
-
   /// language_mode: ID_java, ID_C or another language identifier
   /// if we know the source language in use, irep_idt() otherwise.
   irep_idt language_mode;
 
 protected:
   const bool constant_propagation;
+  const bool self_loops_to_assumptions;
 
   /// The symbol table associated with the goto-program that we're
   /// executing. This symbol table will not additionally contain objects