planner, parser: add basic preparations for intersection type IndexMerge

[time-and-fate]

What problem does this PR solve?

Issue Number: ref #39335

Problem Summary:

Prepare the code for the introduction of intersection type IndexMerge.

What is changed and how it works?

• Move and introduce some util functions.
• Add some nil and length checks.
• Add intersection type IndexMerge related fields to copTask, PhysicalIndexMergeReader, and AccessPath.

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:

• chrysan
• guo-shaoge

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[time-and-fate]

/merge

[ti-chi-bot]

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

Commit hash: 1134814

[time-and-fate]

/run-unit-test

[sre-bot]

TiDB MergeCI notify

🔴 Bad News! New failing [1] after this pr merged.
These new failed integration tests seem to be caused by the current PR, please try to fix these new failed integration tests, thanks!

CI Name	Result	Duration	Compare with Parent commit
idc-jenkins-ci-tidb/sqllogic-test-1	🟥	1 min 20 sec	New failing
idc-jenkins-ci/integration-cdc-test	🟢 all 39 tests passed	24 min	Existing passed
idc-jenkins-ci-tidb/integration-common-test	🟢 all 17 tests passed	15 min	Existing passed
idc-jenkins-ci-tidb/common-test	🟢 all 11 tests passed	9 min 51 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-2	🟢 all 28 tests passed	6 min 9 sec	Existing passed
idc-jenkins-ci-tidb/tics-test	🟢 all 1 tests passed	5 min 53 sec	Existing passed
idc-jenkins-ci-tidb/mybatis-test	🟢 all 1 tests passed	5 min 49 sec	Existing passed
idc-jenkins-ci-tidb/integration-ddl-test	🟢 all 6 tests passed	4 min 51 sec	Existing passed
idc-jenkins-ci-tidb/integration-compatibility-test	🟢 all 1 tests passed	2 min 48 sec	Existing passed
idc-jenkins-ci-tidb/plugin-test	🟢 build success, plugin test success	4min	Existing passed