simplify bitxnor

[kroening]

This adds support for simplifying bitxnor expressions.

• 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).
• 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 58.33333% with 5 lines in your changes missing coverage. Please review.

Project coverage is 78.51%. Comparing base (0db546b) to head (8f55154).
Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/util/bitvector_expr.h	0.00%	3 Missing ⚠️
src/util/simplify_expr_int.cpp	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8506      +/-   ##
===========================================
- Coverage    78.76%   78.51%   -0.25%     
===========================================
  Files         1728     1728              
  Lines       198925   199554     +629     
  Branches     18313    18326      +13     
===========================================
+ Hits        156686   156687       +1     
- Misses       42239    42867     +628     

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