Use bv_utilst::sign_bit

[tautschnig]

We were still repeatedly using bv[bv.size() - 1] in place, when using sign_bit(bv) adds clarity and avoids having to understand encoding details. Also, use .back() to avoid unnecessary repeat arithmetic (which the compiler may or may not optimise away).

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[codecov]

Codecov Report

❌ Patch coverage is 72.22222% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.47%. Comparing base (46f9d54) to head (3fd45d8).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/solvers/flattening/bv_utils.cpp	68.75%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8716      +/-   ##
===========================================
- Coverage    80.47%   80.47%   -0.01%     
===========================================
  Files         1697     1697              
  Lines       208427   208424       -3     
  Branches        73       73              
===========================================
- Hits        167735   167732       -3     
  Misses       40692    40692              

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