[BugFix][TIRx] Keep unsigned branch bounds conservative

[anerli]

Problem and root cause

Access-region analysis can crash while solving a branch condition involving an unsigned variable, for example when the solver constructs a negative unsigned literal. The linear solver subtracts terms and constructs negative coefficients, but unsigned arithmetic is modular and does not satisfy the signed-integer assumptions used by this path.

Change

• src/tirx/transform/ir_utils.cc: admit only signed scalar integer variables to this conditional-bounds solver. Leave other conditions unresolved so access analysis retains conservative bounds.
• tests/python/s_tir/analysis/test_sblock_access_region.py: add ten unsigned cases covering uint32/uint64 equality, inequality, ordering, and wraparound, plus a signed control case confirming that signed bounds still refine.

This deliberately trades refinement of unsupported conditions for correctness.

Tests and validation

All 11 new cases pass. Eight unsigned cases crash on unmodified upstream. The whole test file produces 21 passed, 2 failed, 0 skipped on this branch. Both remaining failures also occur on unmodified upstream with the same configuration:

• test_block_access_region_detector: structural-equality mismatch in the inferred region.
• test_opaque_access_with_tvm_access_ptr: expected ValueError is not raised.

PYTHONPATH=$PWD/python python -m pytest -q \
  tests/python/s_tir/analysis/test_sblock_access_region.py
pre-commit run --files src/tirx/transform/ir_utils.cc \
  tests/python/s_tir/analysis/test_sblock_access_region.py

Validation environment: Apple M4 Max, macOS/Darwin 24.5.0 arm64, Python 3.12.11. Each branch was rebuilt separately from upstream 907a88c87 with:

cmake -S . -B build -G Ninja -DCMAKE_BUILD_TYPE=Release \
  -DUSE_METAL=ON -DUSE_LLVM=OFF -DUSE_CUDA=OFF -DUSE_Z3=OFF
cmake --build build --target tvm tvm_compiler tvm_runtime -j 8

git diff --check passes. The applicable formatting and other pre-commit hooks pass. The repository-wide ASF-header hook reports 11 existing violations, reproduced on unmodified upstream; none is in the changed files.

Related

No duplicate open PR was found. This conditional-solver change is independent of the analyzer work in #67 and #72.