Field sensitivity: account for array size in all index expressions

[tautschnig]

When code outside the core field-sensitivity implementation produces index expressions we must not blindly turn such expressions into ssa_exprt.

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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).
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• 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 62.50000% with 15 lines in your changes missing coverage. Please review.

Project coverage is 78.82%. Comparing base (c66ffbe) to head (40bca65).

Files with missing lines	Patch %	Lines
src/goto-symex/field_sensitivity.cpp	62.50%	15 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8579      +/-   ##
===========================================
- Coverage    78.85%   78.82%   -0.04%     
===========================================
  Files         1732     1732              
  Lines       199119   199236     +117     
  Branches     18560    18564       +4     
===========================================
+ Hits        157023   157038      +15     
- Misses       42096    42198     +102     

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