planner: plan cache statistic result is not correct.

[danmay319]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

The bug can be reproduced by executing following SQLs.

use test;
drop table t;
create table t(a int);
Prepare stmt from "select * from t";
execute stmt;
select EXEC_COUNT,plan_cache_hits, plan_in_cache from information_schema.statements_summary where digest_text='select * from t';
execute stmt;
select EXEC_COUNT,plan_cache_hits, plan_in_cache from information_schema.statements_summary where digest_text='select * from t';
prepare stmt from "select * from t";
execute stmt;
select @@last_plan_from_cache;
select EXEC_COUNT,plan_cache_hits, plan_in_cache from information_schema.statements_summary where digest_text='select * from t';

2. What did you expect to see? (Required)

the `plan_in_cache` of last statement's result is 0 

3. What did you see instead (Required)

the `plan_in_cache` of last statement's result is 1

4. Affected version (Required)

v4.0.0

5. Root Cause Analysis

planInCache should be cleared if PlanInCache is false.