planner: fix same index read plan but has different plan digest (#31560)

[ti-srebot]

cherry-pick #31560 to release-5.4
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/31654

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

git push [email protected]:ti-srebot/tidb.git pr/31654:release-5.4-0a3bcc689f27

---

Signed-off-by: crazycs520 [email protected]

What problem does this PR solve?

Issue Number: close #31438

Check List

Tests

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

Side effects

N/A

Release note

None

[ti-chi-bot]

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[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.

To merge this cherry pick, it must first be approved by the collaborators.

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[ti-srebot]

/run-all-tests

[ti-srebot]

@crazycs520 you're already a collaborator in bot's repo.

[zimulala]

/run-mysql-test tidb-test=pr/1498

[crazycs520]

/run-all-tests