Implement __builtin_ctz{,l,ll} via count_trailing_zeros_exprt

[tautschnig]

Marked as Draft as it depends on #5912.

__builtin_ctz returns the number of trailing zeros. Just like ffs and clz, introduce a new bitvector expression.

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• 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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[codecov]

Codecov Report

Merging #5881 (730c461) into develop (0e34169) will increase coverage by 0.00%.
The diff coverage is 42.76%.

@@            Coverage Diff            @@
##           develop    #5881    +/-   ##
=========================================
  Coverage    74.25%   74.25%            
=========================================
  Files         1445     1445            
  Lines       157479   157635   +156     
=========================================
+ Hits        116933   117059   +126     
- Misses       40546    40576    +30     

Impacted Files	Coverage Δ
src/ansi-c/expr2c.cpp	58.48% <ø> (ø)
src/util/format_expr.cpp	84.87% <0.00%> (-0.79%)	⬇️
src/util/pointer_expr.cpp	72.82% <0.00%> (-18.96%)	⬇️
src/util/pointer_expr.h	77.34% <0.00%> (-22.66%)	⬇️
src/util/simplify_expr_class.h	100.00% <ø> (ø)
...riable-sensitivity/abstract_object/index_range.cpp	100.00% <ø> (ø)
src/solvers/flattening/bv_pointers.cpp	81.26% <13.04%> (-3.36%)	⬇️
src/ansi-c/c_typecheck_expr.cpp	75.17% <75.00%> (-0.01%)	⬇️
src/util/simplify_expr.cpp	76.25% <76.92%> (-8.24%)	⬇️
src/util/bitvector_expr.h	93.07% <90.00%> (-5.97%)	⬇️
... and 49 more

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[martin-cs]

How does this relate to #5879 ?

[tautschnig]

How does this relate to #5879 ?

There is a code overlap in goto_checkt that in principle I could resolve to make them entirely independent, but I'm not too keen on first effectively creating a future merge conflict and later resolving that.

[kroening]

Does it make sense to use the lowering in the simplifier to avoid the duplicate implementation?

[tautschnig]

We could, but it's probably more expensive. Whether that's a practical concern, however, is a different question. I suppose ctz will be fairly rare.