statistics: fix a calculation logic flaw in (*Index).GetRowCount

[time-and-fate]

What problem does this PR solve?

Issue Number: ref #39593

Problem Summary:

This change should be in #40472 but we missed it.

But the cherry-pick PRs of #40472 don't have this problem so we don't need to cherry-pick this PR.

What is changed and how it works?

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

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Reminiscent
• winoros

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[winoros]

/merge

[ti-chi-bot]

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

Commit hash: a9503cf

[ti-chi-bot]

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