Perform simplification of not_exprt as preorder step

[tautschnig]

Each of the possible simplifications has an impact on its operands, which in turn mostly needs to be simplified. Instead of doing this again (as the operand has already been simplified), do most of the work as a preorder step so that only the modified operand is subject to simplification.

We may wish to merge #6118 first to then use resultt as return types and pass const arguments.

• Each commit message has a non-empty body, explaining why the change was made.
• 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 97.22222% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 78.38%. Comparing base (1ed7b2f) to head (9be04c0).

Files	Patch %	Lines
src/util/simplify_expr_boolean.cpp	97.05%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7996      +/-   ##
===========================================
- Coverage    78.38%   78.38%   -0.01%     
===========================================
  Files         1720     1720              
  Lines       187982   188030      +48     
  Branches     18474    18457      -17     
===========================================
+ Hits        147346   147383      +37     
- Misses       40636    40647      +11     

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

[tautschnig]

I have now rebased this to make good use of simplify_exprt::resultt.