hdl._nir: speed up combinational cycle detection.

[wanda-phi]

Fixes #1628.

[codecov]

Codecov Report

❌ Patch coverage is 97.72727% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.26%. Comparing base (ff44683) to head (f0b4e96).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
amaranth/hdl/_nir.py	97.72%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1632      +/-   ##
==========================================
+ Coverage   91.23%   91.26%   +0.02%     
==========================================
  Files          44       44              
  Lines       11402    11444      +42     
  Branches     2224     2230       +6     
==========================================
+ Hits        10403    10444      +41     
- Misses        836      837       +1     
  Partials      163      163              

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