backend: (x86) use named register constants in ops file

[superlopuh]

Avoids spurious register creation, and minimises the init API usage, which I propose changing in a different PR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.16%. Comparing base (592ae08) to head (05ee959).
Report is 12 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3985   +/-   ##
=======================================
  Coverage   90.16%   90.16%           
=======================================
  Files         458      458           
  Lines       58287    58291    +4     
  Branches     5691     5691           
=======================================
+ Hits        52555    52559    +4     
  Misses       4340     4340           
  Partials     1392     1392           

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