Simplify typecast of zero-extension to just a typecast

[tautschnig]

The intervening zero-extension does not alter the semantics and hinders the application of skip_typecast.

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[codecov]

Codecov Report

Attention: Patch coverage is 32.43243% with 25 lines in your changes missing coverage. Please review.

Project coverage is 78.48%. Comparing base (66004dc) to head (b5562a7).

Files with missing lines	Patch %	Lines
src/util/simplify_expr.cpp	32.43%	25 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8586      +/-   ##
===========================================
- Coverage    78.71%   78.48%   -0.23%     
===========================================
  Files         1732     1732              
  Lines       199536   200129     +593     
  Branches     18281    18248      -33     
===========================================
+ Hits        157057   157069      +12     
- Misses       42479    43060     +581     

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[kroening]

Sure this is sound as is?
E.g., an enlarging cast to a signed type would yield a different result if done before vs. after the zero extension.

[tautschnig]

Sure this is sound as is? E.g., an enlarging cast to a signed type would yield a different result if done before vs. after the zero extension.

Thank you, you are of course right. I have reworked the change to address this.