Solver stats reporting: fix expression mapping for guards

[tautschnig]

When converting guards we convert SSA_stept::guard, and not SSA_stept::cond_expr. This needs to be accounted for in solver stats reporting as we would otherwise misleadingly report that expressions like "true" yield a non-empty set of clauses.

Fixes: #6333

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[codecov]

Codecov Report

Base: 78.25% // Head: 78.26% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (557624c) compared to base (d2d3142).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7309   +/-   ##
========================================
  Coverage    78.25%   78.26%           
========================================
  Files         1642     1642           
  Lines       189826   189830    +4     
========================================
+ Hits        148552   148568   +16     
+ Misses       41274    41262   -12     

Impacted Files	Coverage Δ
...instrument/contracts/dynamic-frames/dfcc_utils.cpp	95.73% <100.00%> (+0.03%)	⬆️
.../contracts/dynamic-frames/dfcc_wrapper_program.cpp	98.36% <100.00%> (ø)
src/goto-symex/symex_target_equation.cpp	95.38% <100.00%> (+0.02%)	⬆️
src/goto-symex/solver_hardness.cpp	59.00% <0.00%> (+6.00%)	⬆️

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