Simplify WITH expression over structs with a single component

[tautschnig]

When updating the sole member of a struct we can just use the new value.

• 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.

[peterschrammel]

Is there a test that exercises this case?

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.93%. Comparing base (4f59921) to head (b87c98a).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/util/simplify_expr.cpp	88.88%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8724      +/-   ##
===========================================
- Coverage    79.94%   79.93%   -0.01%     
===========================================
  Files         1698     1698              
  Lines       187677   187681       +4     
  Branches        73       73              
===========================================
- Hits        150032   150022      -10     
- Misses       37645    37659      +14     

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[tautschnig]

Is there a test that exercises this case?

While there already was a test that made us reach the newly added statements I have now also added a check in a .desc file.