Program dependence graph: track SET_RETURN_VALUE instruction

[tautschnig]

We did track a dependency from END_FUNCTION towards the call site when a
return value was being assigned, but failed to keep track of the data
dependency between SET_RETURN_VALUE and END_FUNCTION.

Also clean up commented-out code that used to handle return values, but
has since been superseded.

• 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

Merging #6711 (8305936) into develop (6594cbb) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #6711   +/-   ##
========================================
  Coverage    77.15%   77.15%           
========================================
  Files         1582     1582           
  Lines       182628   182647   +19     
========================================
+ Hits        140900   140920   +20     
+ Misses       41728    41727    -1     

Impacted Files	Coverage Δ
src/analyses/reaching_definitions.cpp	77.51% <ø> (ø)
src/analyses/dependence_graph.cpp	90.36% <100.00%> (+0.68%)	⬆️
src/analyses/dependence_graph.h	85.13% <100.00%> (+1.31%)	⬆️
src/goto-checker/properties.cpp	80.53% <100.00%> (+0.35%)	⬆️
src/util/xml_irep.cpp	25.92% <0.00%> (+1.85%)	⬆️

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