planner: fix union statements order (#8214)

[mtunique]

cherry-pick #8214

[sre-bot]

Hi contributor, thanks for your PR.

This patch needs to be approved by someone of admins. They should reply with "/ok-to-test" to accept this PR for running test automatically.

[zz-jason]

LGTM

[winoros]

FAIL: logical_plan_test.go:1428: testPlanSuite.TestUnion

logical_plan_test.go:1491:
    c.Assert(err, NotNil)
... value = nil

Please check this error.

[zz-jason]

@mtunique CI failed:

FAIL: logical_plan_test.go:1428: testPlanSuite.TestUnion

logical_plan_test.go:1498:
    c.Assert(ToString(p), Equals, tt.best, comment)
... obtained string = "UnionAll{UnionAll{Dual->Projection->Dual->Projection}->Aggr(firstrow(a))->Projection->Dual->Projection}->Projection->Sort"
... expected string = "UnionAll{UnionAll{Dual->Projection->Projection->Dual->Projection->Projection}->Aggr(firstrow(a))->Projection->Dual->Projection->Projection}->Projection->Sort"
... case:5 sql:select * from (select 1 as a  union select 1 union all select 2) t order by a

[XuHuaiyu]

/run-all-tests tidb-test=release-2.0 tikv=release-2.0 pd=release-2.0

[winoros]

lgtm

[mtunique]

/run-all-tests tidb-test=release-2.0 tikv=release-2.0 pd=release-2.0