Deprecate non-const byte_update_exprt accessors

The non-const accessors which have the same name as the const one
shouldn't be used anymore to avoid confusion and unknowningly calling
the non-const version.

Author: romainbrenguier

src/util/byte_operators.h

@@ -87,9 +87,13 @@ class byte_update_exprt : public ternary_exprt
   {
   }
 
+  DEPRECATED(SINCE(2019, 5, 21, "use set_op or as_const instead"))
   exprt &op() { return op0(); }
+  DEPRECATED(SINCE(2019, 5, 21, "use set_offset or as_const instead"))
   exprt &offset() { return op1(); }
+  DEPRECATED(SINCE(2019, 5, 21, "use set_value or as_const instead"))
   exprt &value() { return op2(); }
+
   void set_op(exprt e)
   {
     op0() = std::move(e);