Cumulative fix-up

Will be squashed.

Author: petr-bauch

src/analyses/goto_check.cpp

@@ -1556,7 +1556,7 @@ void goto_checkt::check_rec_if(const exprt &expr, guardt &guard)
   const if_exprt &if_expr = to_if_expr(expr);
 
   INVARIANT(
-    !if_expr.cond().is_boolean(),
+    if_expr.cond().is_boolean(),
     "first argument of if must be boolean, but got " + if_expr.cond().pretty());
 
   check_rec(if_expr.cond(), guard);