relax bitvector constant check for Verilog bitvectors

[kroening]

Verilog bitvectors use a four-valued logic, and hence do not meet the requirements of their binary counterparts.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2d96b83) 79.68% compared to head (85bd173) 79.68%.
Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8191   +/-   ##
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  Coverage    79.68%   79.68%           
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  Files         1681     1681           
  Lines       195296   195296           
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  Hits        155624   155624           
  Misses       39672    39672           

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