executor, planner: refine merge join compare methods to avoid some rare cases

[zz-jason]

What problem does this PR solve?

fix merge join incorrect result in some rare cases.

What is changed and how it works?

change the compare method

Check List

Tests

• Unit test

Code changes

• Has exported function/method change

Related changes

• Need to cherry-pick to the release branch

[zz-jason]

/run-all-tests

[codecov-io]

Codecov Report

Merging #9390 into master will increase coverage by <.01%.
The diff coverage is 73.17%.

@@            Coverage Diff             @@
##           master    #9390      +/-   ##
==========================================
+ Coverage   67.19%   67.19%   +<.01%     
==========================================
  Files         373      373              
  Lines       78331    78386      +55     
==========================================
+ Hits        52634    52671      +37     
- Misses      20985    20995      +10     
- Partials     4712     4720       +8

Impacted Files	Coverage Δ
planner/core/physical_plans.go	60.86% <ø> (ø)	⬆️
executor/builder.go	83.69% <100%> (+0.08%)	⬆️
planner/core/exhaust_physical_plans.go	92.4% <57.14%> (-1.69%)	⬇️
expression/builtin_compare.go	73.07% <74.54%> (-0.84%)	⬇️
executor/merge_join.go	79.45% <75%> (-1.01%)	⬇️
executor/distsql.go	72.24% <0%> (+0.45%)	⬆️
executor/join.go	79.42% <0%> (+0.52%)	⬆️
expression/schema.go	94.53% <0%> (+0.78%)	⬆️
ddl/delete_range.go	77.77% <0%> (+2.64%)	⬆️
store/tikv/scan.go	77.31% <0%> (+3.36%)	⬆️

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[alivxxx]

LGTM

[XuHuaiyu]

LGTM