Optimizer: do not simplify != NULL

[ghazalfamilyusa]

Issue Number: Ref #43407

Problem Summary:

Predicate simplification merges x != constant1 with x in (list of values). Merge is not correct if constant is NULL.
TiDB follows mysql semantics where x != NULL is always false.

Solution
Do not apply the merge of <> and in list for x != NULL.

Tests

Unit tests

Side effects

None

Documentation

None

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• chrysan
• winoros

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[ghazalfamilyusa]

/test check-dev2

[winoros]

/merge

[ti-chi-bot]

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

Commit hash: 0fc9079

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #43442.