Improve bv_utilst less-than-or-less-or-equals

[tautschnig]

1. Duplicate some code to specialise it for signed/unsigned and, thereby, add clarity.
2. De-duplicate code to handle all cases directly in lt_or_le.
3. Make sure we constant-propagate whatever is possible in unsigned comparison to avoid introducing unnecessary fresh literals. Previously, we would create fresh literals even when comparing constant values.

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• 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).
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• 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

❌ Patch coverage is 98.07692% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.48%. Comparing base (46f9d54) to head (e73c451).
⚠️ Report is 4 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/solvers/flattening/bv_utils.cpp	98.07%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8718      +/-   ##
===========================================
+ Coverage    80.47%   80.48%   +0.01%     
===========================================
  Files         1697     1697              
  Lines       208427   208449      +22     
  Branches        73       73              
===========================================
+ Hits        167735   167776      +41     
+ Misses       40692    40673      -19     

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