Value set: lift offset from numeric constants to expressions

[tautschnig]

We can safely track arbitrary expressions as pointer offsets rather than limit ourselves to just constant offsets (and then treating all other expressions as "unknown").

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• 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).
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[codecov]

Codecov Report

Attention: Patch coverage is 73.68421% with 15 lines in your changes missing coverage. Please review.

Project coverage is 80.37%. Comparing base (f1faa61) to head (41811ba).

Files with missing lines	Patch %	Lines
src/pointer-analysis/value_set.cpp	72.72%	15 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8647      +/-   ##
===========================================
- Coverage    80.37%   80.37%   -0.01%     
===========================================
  Files         1686     1686              
  Lines       206889   206894       +5     
  Branches        73       73              
===========================================
- Hits        166289   166283       -6     
- Misses       40600    40611      +11     

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