Fix use of as_const in std_expr

Using as_const() on op() is too late, as op() itself would already
perform non-const operations. This is a fix-up to c267421.

Author: tautschnig

src/util/std_expr.h

@@ -3168,7 +3168,7 @@ class not_exprt:public unary_exprt
 public:
   explicit not_exprt(exprt _op) : unary_exprt(ID_not, std::move(_op))
   {
-    PRECONDITION(as_const(op()).type().id() == ID_bool);
+    PRECONDITION(as_const(*this).op().type().id() == ID_bool);
   }
 
   DEPRECATED("use not_exprt(op) instead")