planner: fix topn wrongly pushed to index scan side when it's a prefix index (#29778)

[ti-srebot]

cherry-pick #29778 to release-5.3
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/29800

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git push git@github.com:ti-srebot/tidb.git pr/29800:release-5.3-a0d3f4fe540c

---

What problem does this PR solve?

Issue Number: close #29711

Problem Summary:

When the IndexScan is using a prefix index, and the TopN.ByItems used the prefix column, the TopN is wrongly pushed to indexPlan.

What is changed and how it works?

Check and push TopN to tablePlan in the above case.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• winoros
• xuyifangreeneyes

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[ti-srebot]

/run-all-tests

[ti-srebot]

@time-and-fate you're already a collaborator in bot's repo.

[zhouqiang-cl]

/merge

[ti-chi-bot]

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

Commit hash: 5f11522