GCC switch range: don't enumerate elements

[tautschnig]

Enumerating large ranges takes a long time or results in exhausting
resources. Generate less-or-equal pairs instead.

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[codecov]

Codecov Report

Merging #5724 (192ebd7) into develop (932599e) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #5724      +/-   ##
===========================================
- Coverage    69.51%   69.51%   -0.01%     
===========================================
  Files         1243     1243              
  Lines       100695   100697       +2     
===========================================
+ Hits         69994    69995       +1     
- Misses       30701    30702       +1     

Flag	Coverage Δ
cproversmt2	43.28% <100.00%> (+<0.01%)	⬆️
regression	66.40% <100.00%> (-0.01%)	⬇️
unit	32.25% <45.45%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
src/goto-programs/goto_convert.cpp	90.03% <100.00%> (-0.15%)	⬇️

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