expression: fix error PushDownNot for contiguous NOT

[XuHuaiyu]

What problem does this PR solve?

Issue Number: close #15725

Problem Summary:
The issue exposes 2 problems:

1. double negation is handled wrongly in PushDownNot. This problem only appears in release-3.0 and release-3.1 because of problem 2.

2. If not appears even number of times in an expression tree, the not can not be eliminated successfully. (not not not not a) can be optimized to (not not a) here.

tidb> desc select * from t where (not not not not a)=a;
+-------------------------+---------+-----------+---------------+--------------------------------------------+
| id                      | estRows | task      | access object | operator info                              |
+-------------------------+---------+-----------+---------------+--------------------------------------------+
| TableReader_7           | 0.80    | root      |               | data:Selection_6                           |
| └─Selection_6           | 0.80    | cop[tikv] |               | eq(not(not(not(not(test.t.a)))), test.t.a) |
|   └─TableFullScan_5     | 1.00    | cop[tikv] | table:t       | keep order:false, stats:pseudo             |
+-------------------------+---------+-----------+---------------+--------------------------------------------+
3 rows in set (0.00 sec)

What is changed and how it works?

Proposal: xxx

What's Changed:
Do not eliminate the innermost NOT.

How it Works:

Related changes

N/A
I'll push specified PR to release branch.

Check List

Tests

• Unit test
• Integration test

Side effects

N/A

Release note

[codecov]

Codecov Report

Merging #16108 into master will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master     #16108   +/-   ##
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  Coverage   80.3807%   80.3807%           
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  Files           506        506           
  Lines        136514     136514           
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  Hits         109731     109731           
  Misses        18223      18223           
  Partials       8560       8560           

[XuHuaiyu]

#16094 is the same bug-fix on release-3.0

[winoros]

lgtm

[eurekaka]

LGTM

[eurekaka]

/merge

[sre-bot]

/run-all-tests

[sre-bot]

/run-all-tests

[sre-bot]

@XuHuaiyu merge failed.

[XuHuaiyu]

/run-all-tests

[XuHuaiyu]

/run-integration-common-test
/run-common-test

[XuHuaiyu]

/run-unit-test

[XuHuaiyu]

unit-test fail because of #15979

[sre-bot]

cherry pick to release-4.0 in PR #16417