bv_utilst::zeros: code cleanup, use more widely

[tautschnig]

Cleanup inspired by #7121. Please review commit-by-commit.

• 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

Merging #7122 (047c065) into develop (c22c61d) will decrease coverage by 0.00%.
The diff coverage is 28.00%.

@@             Coverage Diff             @@
##           develop    #7122      +/-   ##
===========================================
- Coverage    77.92%   77.91%   -0.01%     
===========================================
  Files         1576     1576              
  Lines       181647   181662      +15     
===========================================
  Hits        141542   141542              
- Misses       40105    40120      +15     

Impacted Files	Coverage Δ
src/ansi-c/c_typecheck_expr.cpp	76.92% <6.25%> (-0.50%)	⬇️
src/solvers/flattening/bv_utils.cpp	78.33% <25.00%> (-0.08%)	⬇️
src/solvers/flattening/boolbv_div.cpp	89.28% <100.00%> (-0.37%)	⬇️
src/solvers/flattening/bv_utils.h	86.66% <100.00%> (+0.95%)	⬆️

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