dialect: (rv64) add rv64.GetRegisterOp

[osmanyasar05]

This PR introduces rv64.GetRegisterOp operation to rv64 dialect.

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.25%. Comparing base (9a3c91b) to head (5ea0eec).

Files with missing lines	Patch %	Lines
xdsl/transforms/canonicalization_patterns/riscv.py	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5657   +/-   ##
=======================================
  Coverage   86.25%   86.25%           
=======================================
  Files         403      403           
  Lines       56852    56859    +7     
  Branches     6556     6558    +2     
=======================================
+ Hits        49038    49046    +8     
+ Misses       6283     6282    -1     
  Partials     1531     1531           

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[osmanyasar05]

could not make sure how to duplicate this for rv64. should we create a new folder /tests/backend/rv{32/64}?
one option could be just creating 2 new files, tests/backend/riscv/test_preallocated_rv{32/64}.py ?

[superlopuh]

Fantastic, thank you

[superlopuh]

Code looks good, happy to approve once CI is fixed