Do pretty printing for debug logging only when verbosity is high in incremental SMT2 decision procedure

[thomasspriggs]

This avoids wasting execution time in irept::pretty in the case where
verbosity is set to the default level. Before this performance fix, the
pretty printed output was generated and then discarded when the
verbosity was set to the default level.

Comparing this change against the develop branch, using a benchmark
has been shown to reduce the runtime from approximately 12 seconds to
approximately 3 seconds for that particular example.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• 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).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6704 (2cad881) into develop (e244103) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #6704   +/-   ##
========================================
  Coverage    77.14%   77.15%           
========================================
  Files         1582     1582           
  Lines       182623   182628    +5     
========================================
+ Hits        140892   140900    +8     
+ Misses       41731    41728    -3     

Impacted Files	Coverage Δ
src/goto-programs/goto_convert.cpp	92.33% <ø> (-0.01%)	⬇️
...ncremental/smt2_incremental_decision_procedure.cpp	94.01% <100.00%> (+0.22%)	⬆️
src/util/signal_catcher.cpp	96.15% <0.00%> (+11.53%)	⬆️

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