SMT2: fix pointer arithmetic

[kroening]

This fixes the encoding of pointer arithmetic in the SMT2 backend. Addition now no longer overflows from the offset into the object part.

• 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.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Base: 78.28% // Head: 78.28% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (ad1812a) compared to base (d7dd40a).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7337   +/-   ##
========================================
  Coverage    78.28%   78.28%           
========================================
  Files         1642     1642           
  Lines       189995   190010   +15     
========================================
+ Hits        148737   148757   +20     
+ Misses       41258    41253    -5     

Impacted Files	Coverage Δ
src/solvers/flattening/boolbv_equality.cpp	63.63% <ø> (ø)
src/solvers/lowering/byte_operators.cpp	92.71% <ø> (-0.06%)	⬇️
src/util/byte_operators.h	92.72% <ø> (ø)
src/solvers/smt2/smt2_conv.cpp	66.79% <100.00%> (+0.40%)	⬆️
src/util/byte_operators.cpp	93.10% <100.00%> (+3.10%)	⬆️
src/cprover/propagate.cpp	65.85% <0.00%> (-8.44%)	⬇️

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[tautschnig]

@kroening This also seems to fix pointer-overflow3/no-simplify.desc.