Make guard_exprt::operator-= work with individual conditions

[tautschnig]

Before, operator-= would only apply when both of the operands were
conjunctions. Yet it is perfectly safe use when one or both operands
aren't conjunctions.

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[codecov]

Codecov Report

Merging #5753 (b899cb0) into develop (786f7d7) will increase coverage by 0.00%.
The diff coverage is 91.66%.

@@           Coverage Diff            @@
##           develop    #5753   +/-   ##
========================================
  Coverage    69.62%   69.63%           
========================================
  Files         1243     1243           
  Lines       100812   100823   +11     
========================================
+ Hits         70194    70207   +13     
+ Misses       30618    30616    -2     

Flag	Coverage Δ
cproversmt2	43.36% <50.00%> (+<0.01%)	⬆️
regression	66.54% <91.66%> (+<0.01%)	⬆️
unit	32.21% <0.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
src/analyses/guard_expr.cpp	98.50% <91.66%> (-1.50%)	⬇️
src/goto-symex/show_program.cpp	91.73% <0.00%> (+2.47%)	⬆️

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[martin-cs]

Obviously I wonder "could this go further" but I think the point is to keep it light weight. The BDD patches et al. are probably a better solution than much more explicit case-by-case handling.