*: replace compareDatum by compare and fix wrong optimize order by (#30273)

[ti-srebot]

cherry-pick #30273 to release-5.2
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/32321

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---

Signed-off-by: wjhuang2016 huangwenjun1997@gmail.com

What problem does this PR solve?

Issue Number: close #30271. Related to #29867.

Problem Summary:

1. replace compareDatum by compare
2. isSameValue may use wrong collation

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

fix a bug when reducing order by clause for the index which leads to the wrong result.

[ti-chi-bot]

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[ti-srebot]

/run-all-tests

[ti-srebot]

@wjhuang2016 you're already a collaborator in bot's repo.

[ti-chi-bot]

@ti-srebot: PR needs rebase.

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[winoros]

This one is duplicated. See #30551