boolbv_index: preserve endianness when generating byte extract

[tautschnig]

byte_extract(...)[index] translated to byte_extract expressions with
config-specified endianness rather than preserving the endianness of the
original byte_extract expression. This, however, is a hardly exercised
code path as the simplifier already did a similar transformation that
did preserve endianness.

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• 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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[codecov]

Codecov Report

Merging #6100 (a419041) into develop (4c14789) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #6100      +/-   ##
===========================================
+ Coverage    74.52%   74.61%   +0.08%     
===========================================
  Files         1447     1447              
  Lines       157808   157821      +13     
===========================================
+ Hits        117610   117756     +146     
+ Misses       40198    40065     -133     

Impacted Files	Coverage Δ
src/solvers/flattening/boolbv_index.cpp	89.72% <100.00%> (+10.77%)	⬆️
src/solvers/smt2/smt2_conv.cpp	60.63% <0.00%> (+0.22%)	⬆️
src/solvers/lowering/byte_operators.cpp	92.16% <0.00%> (+0.36%)	⬆️
src/ansi-c/c_typecast.cpp	79.00% <0.00%> (+0.55%)	⬆️
src/solvers/flattening/boolbv_byte_extract.cpp	82.27% <0.00%> (+12.65%)	⬆️
src/solvers/floatbv/float_bv.cpp	88.33% <0.00%> (+14.47%)	⬆️

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[tautschnig]

I have meanwhile found some ways to exercise this code, will merge this PR once I have added tests.