Fix _Bool preincrement

[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

Merging #7049 (4d215fa) into develop (438aad3) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #7049   +/-   ##
========================================
  Coverage    77.88%   77.88%           
========================================
  Files         1569     1569           
  Lines       180989   180995    +6     
========================================
+ Hits        140962   140968    +6     
  Misses       40027    40027           

Impacted Files	Coverage Δ
src/goto-programs/goto_convert_side_effect.cpp	95.21% <100.00%> (+0.07%)	⬆️

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