fix update_bit lowering

[kroening]

• 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).
• n/a 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

Attention: Patch coverage is 96.15385% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.80%. Comparing base (4f56b6a) to head (a42c371).
Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/util/bitvector_expr.cpp	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #8496    +/-   ##
=========================================
  Coverage    78.79%   78.80%            
=========================================
  Files         1727     1728     +1     
  Lines       198783   198853    +70     
  Branches     18502    18317   -185     
=========================================
+ Hits        156638   156700    +62     
- Misses       42145    42153     +8     

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