Perform pointer validity checks when doing pointer arithmetic

[tautschnig]

Arithmetic over pointers requires that they point to valid objects (or
one past the end of an object).

The test uncovered two further problems: 1) there was a typo in
subtraction handling in bv_pointerst; 2) redundant assertions are
removed, even when they refer to different expressions.

Fixes: #5426

• Each commit message has a non-empty body, explaining why the change was made.
• 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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[martin-cs]

Seems pretty good.

[codecov]

Codecov Report

Merging #5844 (3acdb52) into develop (dbbe1da) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #5844      +/-   ##
===========================================
+ Coverage    72.86%   72.87%   +0.01%     
===========================================
  Files         1421     1421              
  Lines       154173   154181       +8     
===========================================
+ Hits        112337   112359      +22     
+ Misses       41836    41822      -14     

Impacted Files	Coverage Δ
src/analyses/goto_check.cpp	88.21% <100.00%> (+0.09%)	⬆️
src/solvers/flattening/bv_pointers.cpp	84.24% <100.00%> (+3.06%)	⬆️

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