Code review comment for lp:~roccoblues/drizzle/port-constants-evalutation-fixes

This ports the fixes for MySQL Bug#33546 to drizzle. Test-suite passes.

One small thing I've noticed is that in the test result file this query

EXPLAIN EXTENDED SELECT * FROM t2 LEFT JOIN t1 ON a = b + 1;

leads to different results in drizzle and mysql:

MySQL:
EXPLAIN EXTENDED SELECT * FROM t2 LEFT JOIN t1 ON a = b + 1;
id select_type table type possible_keys key key_len ref rows filtered Extra
 1 SIMPLE t2 system NULL NULL NULL NULL 1 100.00
 1 SIMPLE t1 ALL NULL NULL NULL NULL 10 100.00 Using where
Warnings:
Note 1003 select '2' AS `b`,`test`.`t1`.`a` AS `a` from `test`.`t1` where 1

Drizzle:
EXPLAIN EXTENDED SELECT * FROM t2 LEFT JOIN t1 ON a = b + 1;
id select_type table type possible_keys key key_len ref rows filtered Extra
1 SIMPLE t2 ALL NULL NULL NULL NULL 1 100.00
1 SIMPLE t1 ALL NULL NULL NULL NULL 10 100.00
Warnings:
Note 1003 select `test`.`t2`.`b` AS `b`,`test`.`t1`.`a` AS `a` from `test`.`t2` left join `test`.`t1` on((`test`.`t1`.`a` = (`test`.`t2`.`b` + 1))) where 1

I don't think that's because of my changes but please double check. Jay said on IRC: "might be some constant propogation in the optimizer that is not in Drizzle. I wouldn't worry about it too much. The case that is being optimized is pretty minimal."