Simplify non-trivial pointer subtraction

[tautschnig]

We can also simplify the case where both pointers refer to the same object, as long as the pointers are at most one element beyond bounds.

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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/
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[tautschnig]

Requires #7338 for tests to pass.

[codecov]

Codecov Report

Base: 78.29% // Head: 78.31% // Increases project coverage by +0.02% 🎉

Coverage data is based on head (633875b) compared to base (fb99063).
Patch coverage: 95.96% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7341      +/-   ##
===========================================
+ Coverage    78.29%   78.31%   +0.02%     
===========================================
  Files         1642     1644       +2     
  Lines       190023   190226     +203     
===========================================
+ Hits        148769   148980     +211     
+ Misses       41254    41246       -8     

Impacted Files	Coverage Δ
src/solvers/smt2_incremental/object_tracking.h	100.00% <ø> (ø)
..._incremental/smt2_incremental_decision_procedure.h	75.00% <ø> (ø)
...c/goto-programs/ensure_one_backedge_per_target.cpp	84.09% <82.97%> (-1.63%)	⬇️
...t/solvers/smt2_incremental/convert_expr_to_smt.cpp	99.60% <94.73%> (-0.10%)	⬇️
src/solvers/smt2/smt2_conv.cpp	66.97% <95.83%> (+0.18%)	⬆️
src/goto-programs/builtin_functions.cpp	58.95% <100.00%> (+0.43%)	⬆️
src/goto-programs/remove_returns.cpp	98.15% <100.00%> (ø)
src/goto-symex/symex_clean_expr.cpp	93.16% <100.00%> (ø)
src/goto-symex/symex_dereference.cpp	89.89% <100.00%> (+0.10%)	⬆️
...c/solvers/smt2_incremental/convert_expr_to_smt.cpp	83.38% <100.00%> (+0.47%)	⬆️
... and 12 more

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[peterschrammel]

Have we got a test for this case?

[tautschnig]

Have we got a test for this case?

We actually do, because tests failed here when #7338 wasn't yet merged. Also, Pointer_difference2 failed when my initial version had not properly taken into account object bounds.