[release-21.0] evalengine: normalize types during compilation (#17887)

[vitess-bot]

Description

This is a backport of #17887

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 67.52%. Comparing base (913c838) to head (beaf359).
Report is 1 commits behind head on release-21.0.

Files with missing lines	Patch %	Lines
go/vt/vtgate/evalengine/expr_bvar.go	40.00%	3 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release-21.0   #17896      +/-   ##
================================================
- Coverage         67.52%   67.52%   -0.01%     
================================================
  Files              1572     1572              
  Lines            252012   252017       +5     
================================================
- Hits             170173   170162      -11     
- Misses            81839    81855      +16     

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