statistics: fix unnecessary index fmsketch loading (#42074)

[ti-chi-bot]

This is an automated cherry-pick of #42074

What problem does this PR solve?

Issue Number: close #42052

Problem Summary:

What is changed and how it works?

When loading index stats from storage, we don't need to load index fmsketch unless loadAll is true. We only use index fmsketch when merging partition stats to global stats, where loadAll is true.

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]

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[chrysan]

/retest

[ti-chi-bot]

@ti-chi-bot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/check_dev	9f42d4d	link	true	/test check-dev
idc-jenkins-ci-tidb/check_dev_2	9f42d4d	link	true	/test check-dev2
idc-jenkins-ci-tidb/build	9f42d4d	link	true	/test build
idc-jenkins-ci-tidb/unit-test	9f42d4d	link	true	/test unit-test
idc-jenkins-ci-tidb/mysql-test	9f42d4d	link	true	/test mysql-test

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[xuyifangreeneyes]

Since we do some refactors after release-6.5, #42074 cannot be automatically cherry picked. So I manually cherry pick it(#44330).