backend: (csl) added arith.select to csl backend

[dk949]

arith.select can be translated to a CSL ternary operator
if (cond) lhs else rhs.

[dk949]

I thought we might need it. Turns out we didn't really, but I figured I'd push it anyway since it's already written.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.03%. Comparing base (9dc299a) to head (f2fe20e).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3199      +/-   ##
==========================================
- Coverage   90.03%   90.03%   -0.01%     
==========================================
  Files         432      432              
  Lines       54435    54441       +6     
  Branches     8443     8444       +1     
==========================================
+ Hits        49012    49016       +4     
- Misses       4056     4057       +1     
- Partials     1367     1368       +1     

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