Use array comprehension for large byte_extract lowering

[tautschnig]

lower_byte_extract_array_vector expands byte_extract of an array type into an element-by-element array_exprt. For large arrays (e.g., char[100000] in a union), this creates N sub-expressions that are each recursively lowered and simplified, resulting in O(N^2) behavior.

When the array size exceeds MAX_FLATTENED_ARRAY_SIZE (1000), use array_comprehension_exprt instead. This path already exists for arrays with non-constant size; now it is also used for large constant-size arrays. This reduces the lowering from O(N) expressions to O(1).

Performance on union_large_array (char[100000] in a union):
Before: >120s with --arrays-uf-always
After: 2.3s with --arrays-uf-always

Co-authored-by: Kiro

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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).
• My commit message includes data points confirming performance improvements (if claimed).
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[Copilot]

Pull request overview

This PR optimizes lower_byte_extract_array_vector to avoid element-by-element expansion for large constant-sized arrays by switching to an array_comprehension_exprt representation once the array exceeds MAX_FLATTENED_ARRAY_SIZE, reducing expression blow-up and improving lowering performance.

Changes:

• Include magic.h to access MAX_FLATTENED_ARRAY_SIZE.
• Route large constant-sized array byte-extract lowering through array_comprehension_exprt instead of generating N lowered sub-expressions.
• Add comments documenting the O(N²) behavior and rationale for the comprehension-based lowering.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.01%. Comparing base (15eb10a) to head (a31877d).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8840   +/-   ##
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  Coverage    80.00%   80.01%           
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  Files         1700     1700           
  Lines       188252   188254    +2     
  Branches        73       73           
========================================
+ Hits        150613   150629   +16     
+ Misses       37639    37625   -14     

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