dialects: (riscv) Make riscv.get_register pure

[AntonLydike]

This makes get_register pure, as it should be.

The op never reads/writes and is safe to DCE and CSE.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.79%. Comparing base (772df11) to head (613238c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2797      +/-   ##
==========================================
- Coverage   89.79%   89.79%   -0.01%     
==========================================
  Files         387      387              
  Lines       48507    48507              
  Branches     7434     7434              
==========================================
- Hits        43559    43557       -2     
- Misses       3780     3782       +2     
  Partials     1168     1168              

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