compiler: Fix three guard bugs exposed by buffering

[ggorman]

Three guard bugs that buffering exposes, each with a single-writer test against opt='noop':

1. GuardFactorNe was an equality. It inherited CondEq ahead of CondNe through GuardFactor, so negating a subsampling guard reproduced the guard: one equation under Not(GuardFactor(ct)) with factor 2 printed (time)%(ctf) == 0 and wrote f = [1, 0, 3, 0] instead of [0, 2, 0, 4].
2. if (false) was emitted. A condition SymPy folds to false (e.g. time < 0) printed if (false), which is not C99 without stdbool.h: a compile error.
3. {time: true} guards. time >= 0 folds to true and gave a Cluster a guard distinct from no guard, so buffering created one buffer per "guard" for equivalent accesses.

Tests: test_dimension.py::TestConditionalDimension::test_negated_factor_condition, ::test_false_condition, test_buffering.py::test_true_condition_single_buffer. Each fails with only its commit's source change reverted.

First of three stacked PRs: this one, then #3062 (copy-back skip rule) and #3063 (stale reads and lost writes).

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[codecov]

Codecov Report

❌ Patch coverage is 97.87234% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 84.24%. Comparing base (2707ac9) to head (38e0aa4).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
devito/passes/iet/languages/C.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3061      +/-   ##
==========================================
+ Coverage   84.23%   84.24%   +0.01%     
==========================================
  Files         258      258              
  Lines       57070    57112      +42     
  Branches     4874     4875       +1     
==========================================
+ Hits        48072    48115      +43     
  Misses       8170     8170              
+ Partials      828      827       -1     

Flag	Coverage Δ
pytest-gpu-aomp-amdgpuX	68.51% <75.00%> (-0.01%)	⬇️
pytest-gpu-gcc-	79.03% <97.87%> (+0.02%)	⬆️
pytest-gpu-icx-	78.93% <97.87%> (+0.01%)	⬆️
pytest-gpu-nvc-nvidiaX	69.08% <75.00%> (-0.01%)	⬇️

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