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.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[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).
Report is 100 commits behind head on develop.

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     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[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.

[kroening]

I am afraid that I still don't think this is sound in general. Note that a zero extension on a signed type behaves differently from the typecast: a number that was negative before is now positive.

The same applies to the concatenation of a bunch of zeros plus a signed number.

[tautschnig]

Closing as this isn't the right approach: we should instead audit (and possibly replace) all uses of skip_typecast.