mpi: Patch unnecessary halo updates inserted with functions on subdomains

[EdCaunt]

When constructing an operator with a trivial equation in which a field defined on a subdomain is updated without using any surrounding points (i.e. Eq(u.forward, u)), halo exchanges get inserted as the symbolic subdomain offsets do not get evaluated until runtime, and thus the compiler cannot currently determine that it will not need data from within the halo. These halo exchanges are erroneous as they read and write from the same spatial position and thickness values are populated to avoid indexing into the halo.

Note: currently has a lot of comments as there are aspects here I'm not entirely sure about and which need sanity checking by someone with more experience with MPI data dependence

[codecov]

Codecov Report

❌ Patch coverage is 40.00000% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.53%. Comparing base (35f711f) to head (06e1fe7).
⚠️ Report is 28 commits behind head on main.

Files with missing lines	Patch %	Lines
tests/test_mpi.py	14.28%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2986      +/-   ##
==========================================
- Coverage   83.57%   83.53%   -0.05%     
==========================================
  Files         257      257              
  Lines       53783    53917     +134     
  Branches     4608     4614       +6     
==========================================
+ Hits        44951    45038      +87     
- Misses       8035     8078      +43     
- Partials      797      801       +4     

Flag	Coverage Δ
pytest-gpu-aomp-amdgpuX	68.56% <100.00%> (+0.03%)	⬆️
pytest-gpu-gcc-	78.16% <40.00%> (-0.04%)	⬇️
pytest-gpu-icx-	78.09% <40.00%> (-0.06%)	⬇️
pytest-gpu-nvc-nvidiaX	69.21% <100.00%> (+0.02%)	⬆️

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