SMT2: fix extractbit with non-const index

[kroening]

This fixes a constant literal in the encoding of extractbit for the case of a non-const index.

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• 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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[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (5bdff46) 79.69% compared to head (7834962) 79.69%.

Files	Patch %	Lines
src/solvers/smt2/smt2_conv.cpp	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8180      +/-   ##
===========================================
- Coverage    79.69%   79.69%   -0.01%     
===========================================
  Files         1680     1680              
  Lines       195256   195257       +1     
===========================================
  Hits        155617   155617              
- Misses       39639    39640       +1     

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