Use boolean_negate for immediate simplification

[tautschnig]

When the operand can be a Boolean constant we can don't need to wait for simplify_exprt to clean up the avoidable not_exprt.

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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: Patch coverage is 83.33333% with 3 lines in your changes missing coverage. Please review.

Project coverage is 78.35%. Comparing base (2212cd6) to head (f63dfa9).
Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
...s/dynamic-frames/dfcc_contract_clauses_codegen.cpp	50.00%	2 Missing ⚠️
src/goto-instrument/contracts/utils.cpp	80.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8449      +/-   ##
===========================================
- Coverage    78.38%   78.35%   -0.04%     
===========================================
  Files         1726     1726              
  Lines       188616   188695      +79     
  Branches     18295    18301       +6     
===========================================
- Hits        147855   147851       -4     
- Misses       40761    40844      +83     

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