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

[ti-srebot]

cherry-pick #32813 to release-5.2
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/32839

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/tidb.git pr/32839:release-5.2-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]

This pull request has been approved by:

• LittleFall
• windtalker

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[ti-srebot]

/run-all-tests

[ti-srebot]

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

[windtalker]

LGTM

[windtalker]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 0453bd5

[ti-chi-bot]

@ti-srebot: Your PR was out of date, I have automatically updated it for you.

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[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/f918164403d7f616fa304b954fd92c8a31bb8c4f