planner: Fix the issue that TiDB may dispatch duplicated tasks to TiFlash (#32813)

[ti-srebot]

cherry-pick #32813 to release-5.1
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/32838

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git push git@github.com:ti-srebot/tidb.git pr/32838:release-5.1-fcdbf826ea8a

---

What problem does this PR solve?

Issue Number: close #32814, pingcap/tiflash#4163

Problem Summary:

1. if a fragment has two children, one's ExchangeSender is Broadcast Type, another's is Type PassThrough.
2. This fragment should be singleton, but because of the wrong implementation, it isn't.
3. So this fragment will generate two tasks, and the data will be double.

What is changed and how it works?

if there is one child with passthrough exchangeType, the fragment will be singleton.

Check List

Tests

• Unit test

Release note

planner: Fix the issue that TiDB may dispatch duplicated tasks to TiFlash

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• LittleFall

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[ti-srebot]

/run-all-tests

[ti-chi-bot]

@ti-srebot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

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[ti-srebot]

@LittleFall you're already a collaborator in bot's repo.

[VelocityLight]

This pull request is closed because it's related version has closed automatic cherry-picking.
If it's still needed, you can reopen it or just regenerate it using bot,
see:

• https://prow.tidb.io/command-help#cherrypick
• https://book.prow.tidb.net/#/plugins/cherrypicker

You can find more details at:

• https://internals.tidb.io/t/topic/785