planner: refactor a few methods about cost calculation

[qw4990]

What problem does this PR solve?

Issue Number: close #33722

Problem Summary: planner: refactor a few methods about cost calculation

What is changed and how it works?

No logical change, just refactor:

1. create a new dedicated method for IndexLookUp cost calculation;
2. remove task from PhysicalIndexJoin.GetCost;

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

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• time-and-fate
• xuyifangreeneyes

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[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/c151d50f66e0b6df6bc4071f5cd1febe982912ab

[qw4990]

/run-check_dev_2

[qw4990]

/merge

[ti-chi-bot]

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

Commit hash: c151d50

[qw4990]

/run-mysql-test

[hawkingrei]

/run-unit-test