[release-21.0] bugfix: INSERT IGNORE not inserting rows (#18151)

[vitess-bot]

Description

This is a backport of #18151

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.55%. Comparing base (c20263b) to head (4082c09).

Additional details and impacted files

@@               Coverage Diff                @@
##           release-21.0   #18164      +/-   ##
================================================
+ Coverage         67.53%   67.55%   +0.01%     
================================================
  Files              1572     1572              
  Lines            252261   252270       +9     
================================================
+ Hits             170353   170409      +56     
+ Misses            81908    81861      -47     

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