dialects: (riscv, x86) do not print empty <> for unallocated registers

[superlopuh]

I think this removes quite a bit of noise when reading the IR.

Note that the code changes are the three bottom files at the bottom of the files view, the rest is filecheck changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.78%. Comparing base (17ec8ba) to head (4853f11).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2786      +/-   ##
==========================================
- Coverage   89.78%   89.78%   -0.01%     
==========================================
  Files         382      382              
  Lines       48412    48413       +1     
  Branches     7427     7426       -1     
==========================================
- Hits        43469    43468       -1     
- Misses       3780     3781       +1     
- Partials     1163     1164       +1     

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[AntonLydike]

Damn, this does look much cleaner.