planner: reuse slices when to deal with tiflash predicate push down

[hawkingrei]

What problem does this PR solve?

Issue Number: close #xxx

Problem Summary:

What changed and how does it work?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

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

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None

[ti-chi-bot]

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[tiprow]

@hawkingrei: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
fast_test_tiprow	d8173ae	link	true	/test fast_test_tiprow

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[ti-chi-bot]

@hawkingrei: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/build	d8173ae	link	true	/test build
idc-jenkins-ci-tidb/unit-test	d8173ae	link	true	/test unit-test
idc-jenkins-ci-tidb/check_dev_2	d8173ae	link	true	/test check-dev2

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 73.5005%. Comparing base (0f5aa22) to head (d8173ae).

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #61522        +/-   ##
================================================
+ Coverage   73.0925%   73.5005%   +0.4080%     
================================================
  Files          1729       1729                
  Lines        480695     480716        +21     
================================================
+ Hits         351352     353329      +1977     
+ Misses       107799     105855      -1944     
+ Partials      21544      21532        -12     

Flag	Coverage Δ
integration	42.6010% <0.0000%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	52.7804% <ø> (ø)
parser	∅ <ø> (∅)
br	46.9655% <ø> (+0.0078%)	⬆️

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