Simplify index: ensure consistent types

[tautschnig]

When computing a new offset for a byte extract, do not implicitly assume
that byte-extract offsets and array index operations use exactly the
same type for we don't prescribe the type to use as a byte-extract
offset.

Fixes: #6298

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[codecov]

Codecov Report

Merging #6746 (bdf877d) into develop (5c25144) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #6746   +/-   ##
========================================
  Coverage    76.86%   76.86%           
========================================
  Files         1589     1589           
  Lines       183814   183822    +8     
========================================
+ Hits        141293   141302    +9     
+ Misses       42521    42520    -1     

Impacted Files	Coverage Δ
src/util/simplify_expr_array.cpp	90.72% <100.00%> (+0.19%)	⬆️
src/util/simplify_expr_int.cpp	85.59% <100.00%> (+0.09%)	⬆️
src/util/piped_process.cpp	83.13% <0.00%> (+1.20%)	⬆️

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