Fix count-trailing-zeros lowering for value zero

[tautschnig]

The previous implementation would produce an off-by-one error in case of
an argument 0, which count_trailing_zeros is defined for (even when
__builtin_ctz is undefined in that case).

Fixes: #6702

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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/
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• My PR is restricted to a single feature or bugfix.
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[codecov]

Codecov Report

Merging #6703 (e244103) into develop (1154f13) will increase coverage by 0.22%.
The diff coverage is 88.11%.

@@             Coverage Diff             @@
##           develop    #6703      +/-   ##
===========================================
+ Coverage    76.92%   77.14%   +0.22%     
===========================================
  Files         1583     1582       -1     
  Lines       183314   182623     -691     
===========================================
- Hits        141018   140892     -126     
+ Misses       42296    41731     -565     

Impacted Files	Coverage Δ
jbmc/src/java_bytecode/lazy_goto_model.h	100.00% <ø> (+7.40%)	⬆️
jbmc/src/jdiff/jdiff_parse_options.h	100.00% <ø> (ø)
src/cpp/cpp_typecheck_conversions.cpp	85.14% <0.00%> (+0.10%)	⬆️
src/goto-instrument/dump_c.cpp	80.49% <0.00%> (+0.28%)	⬆️
src/goto-instrument/dump_c_class.h	100.00% <ø> (ø)
src/goto-instrument/goto_program2code.cpp	68.84% <0.00%> (+0.06%)	⬆️
src/goto-programs/string_abstraction.cpp	92.45% <ø> (-0.01%)	⬇️
src/goto-programs/xml_expr.cpp	51.21% <0.00%> (ø)
src/util/expr_util.h	100.00% <ø> (ø)
src/util/pointer_predicates.cpp	93.10% <ø> (-0.45%)	⬇️
... and 53 more

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