Flow-insensitive value set: don't create index expressions over non-array objects

[tautschnig]

When evaluating an index expression, the value set must ignore pointed-to objects that are neither arrays nor vectors. Any appearance of such objects should produce unknown instead. (Trying to create an index_exprt with neither an array nor a vector as root object will fail an invariant.)

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• 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.
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.36%. Comparing base (beb96b9) to head (0efab54).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8651      +/-   ##
===========================================
- Coverage    80.36%   80.36%   -0.01%     
===========================================
  Files         1686     1686              
  Lines       206938   206940       +2     
  Branches        73       73              
===========================================
- Hits        166304   166303       -1     
- Misses       40634    40637       +3     

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