DFCC instrumentation: skip unused functions

[tautschnig]

Do not attempt to instrument functions that will never be used anyway. As we eventually use remove_unused_functions there is no point in trying to instrument them, and there is a scenario where the function may not have been compiled (see included test).

• Each commit message has a non-empty body, explaining why the change was made.
• 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).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.37%. Comparing base (9db5b1a) to head (0c5644b).
Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
.../goto-instrument/contracts/dynamic-frames/dfcc.cpp	92.30%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8628      +/-   ##
===========================================
- Coverage    80.37%   80.37%   -0.01%     
===========================================
  Files         1686     1686              
  Lines       206872   206885      +13     
  Branches        73       73              
===========================================
+ Hits        166268   166276       +8     
- Misses       40604    40609       +5     

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