Simplify byte extract: support signed and unsigned offsets

[tautschnig]

The minimized example from #7357 resulted in an invariant failure in solvers/flattening/boolbv_add_sub.cpp, reporting "add/sub with mixed types." This was caused by simplifying nested byte-extract operations where one used unsigned offsets and the other one signed. Since we do not required a particular type for byte-extract offsets we must cope with different offset types when folding nested byte-extract operations into a single one.

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[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.17 🎉

Comparison is base (e7787ad) 78.33% compared to head (9df171e) 78.50%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7705      +/-   ##
===========================================
+ Coverage    78.33%   78.50%   +0.17%     
===========================================
  Files         1674     1674              
  Lines       191956   191958       +2     
===========================================
+ Hits        150360   150702     +342     
+ Misses       41596    41256     -340     

Impacted Files	Coverage Δ
src/util/simplify_expr.cpp	85.51% <100.00%> (+0.01%)	⬆️

... and 24 files with indirect coverage changes

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