protect exprt::opX()

This prevents direct access to exprt::opX() -- the expectation is that the
relevant specializations are used, which continue to offer these methods.

Author: kroening

src/util/expr.h

@@ -98,6 +98,7 @@ class exprt:public irept
   const operandst &operands() const
   { return (const operandst &)get_sub(); }
 
+protected:
   exprt &op0()
   { return operands().front(); }
 
@@ -122,6 +123,7 @@ class exprt:public irept
   const exprt &op3() const
   { return operands()[3]; }
 
+public:
   void reserve_operands(operandst::size_type n)
   { operands().reserve(n) ; }