code simplification, fix conflict in new_diseq_eh

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>

src/ast/rewriter/seq_rewriter.h

@@ -379,6 +379,8 @@ public:
         return result;
     }
 
+    expr_ref mk_symmetric_diff(expr *r1, expr *r2);
+
     /**
      * check if regular expression is of the form all ++ s ++ all ++ t + u ++ all, where, s, t, u are sequences
      */