util/ranger: don't merge consecutive ranges when building ranges for cnf item (#41661)

[ti-chi-bot]

This is an automated cherry-pick of #41661

What problem does this PR solve?

Issue Number: close #41572

Problem Summary:

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:

• hawkingrei

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[elsa0520]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: elsa0520, hawkingrei, time-and-fate

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-07-11 10:00:22.772777977 +0000 UTC m=+412514.543116685: ☑️ agreed by elsa0520.
• 2023-07-11 10:05:13.887465847 +0000 UTC m=+412805.657804560: ☑️ agreed by time-and-fate.

[purelind]

/test all

[tiprow]

@purelind: No presubmit jobs available for pingcap/tidb@release-7.1

In response to this:

/test all

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