Symex guards: avoid unnecessary duplicate negation

[tautschnig]

We negated the right-hand side assigned to a guard symbol just so as to then negate the use of the guard symbol. Remove this duplicate negation.

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• 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).
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• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.24%. Comparing base (2d63a71) to head (150402c).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8041   +/-   ##
========================================
  Coverage    78.23%   78.24%           
========================================
  Files         1720     1720           
  Lines       188342   188341    -1     
  Branches     18441    18467   +26     
========================================
+ Hits        147353   147358    +5     
+ Misses       40989    40983    -6     

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