wrong behavior when enabling prepare-cache in stmt order by ?

[winoros]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

create table t1 (a int, b int);
insert into t1 (a,b) values (2,8),(1,9),(3,7);

# Will order by index
prepare stmt from "select * from t1 order by ?";
set @a=1;
execute stmt using @a;
set @a=2;
execute stmt using @a;
deallocate prepare stmt;

2. What did you expect to see?

The results are ordered by column.

3. What did you see instead?

Results are unordered.

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

master dd65caa