compiler: Tweak blocking for CPML-like subdomains and GPUs

[FabioLuporini]

Still things to tweak, but it's a start

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.66%. Comparing base (25c2dc1) to head (49855a0).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3007      +/-   ##
==========================================
+ Coverage   79.35%   83.66%   +4.30%     
==========================================
  Files         257      257              
  Lines       54630    54654      +24     
  Branches     4682     4685       +3     
==========================================
+ Hits        43352    45724    +2372     
+ Misses      10415     8117    -2298     
+ Partials      863      813      -50     

Flag	Coverage Δ
pytest-gpu-aomp-amdgpuX	68.52% <58.62%> (?)
pytest-gpu-gcc-	78.29% <100.00%> (+<0.01%)	⬆️
pytest-gpu-icx-	78.21% <100.00%> (+35.28%)	⬆️
pytest-gpu-nvc-nvidiaX	69.14% <58.62%> (?)

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