Bugfix: flattening for non-binary bitnor, bitnand, bitxnor

[kroening]

This fixes the flattening for bitnor, bitnand, bitxnor for the case that the expression has one operand or more than two operands.

For one operand, the result is now the bit-wise negation.

For three or more operands, the result is now the bit-wise negation of the bitor, bitand, bitxor with the same operands.

• 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 81.25000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 78.55%. Comparing base (c4aaafd) to head (1d9e73b).
Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/solvers/flattening/boolbv_bitwise.cpp	81.25%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8542      +/-   ##
===========================================
- Coverage    78.55%   78.55%   -0.01%     
===========================================
  Files         1729     1729              
  Lines       199714   199722       +8     
  Branches     18312    18312              
===========================================
- Hits        156892   156888       -4     
- Misses       42822    42834      +12     

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