SMT2: allow unary bitand, bitor, bitxor

[kroening]

This converts unary bitand, bitor, bitxor bit-vector expressions as the identity in the SMT2 backend.

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[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.94%. Comparing base (56d8598) to head (fafc39c).
⚠️ Report is 15 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/solvers/smt2/smt2_conv.cpp	80.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8794      +/-   ##
===========================================
+ Coverage    79.93%   79.94%   +0.01%     
===========================================
  Files         1698     1699       +1     
  Lines       187698   187785      +87     
  Branches        73       73              
===========================================
+ Hits        150037   150133      +96     
+ Misses       37661    37652       -9     

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[tautschnig]

Why do we even produce such expressions? I acknowledge that the propositional back-end support this, but shouldn't we instead change the code that attempts to produce those single-operand expressions?

[kroening]

Why do we even produce such expressions? I acknowledge that the propositional back-end support this, but shouldn't we instead change the code that attempts to produce those single-operand expressions?

I considered it -- and went for supporting the unary ones. The idea is that you'd need to handle special cases in a lot of cases where these are produced, as opposed to fairly easy handling in few the places where they are consumed.