[TIRx] Add a dedicated boolean buffer lowering pass

[guan404ming]

Why

The bool→int8 backing-array conversion was duplicated inline across FlattenBuffer and LowerTIRxCleanup.

How

• Add LowerBoolBuffer pass that rewrites bool buffers to int8 and inserts load/store casts.
• Remove the duplicated bool handling from FlattenBuffer and LowerTIRxCleanup.
• Run it after FlattenBuffer (before VectorizeLoop) in every pipeline, with structural and build-and-run tests.

[gemini-code-assist]

Code Review

This pull request introduces a dedicated LowerBoolBuffer pass to replace boolean buffers with an int8 backing array, moving this logic out of FlattenBuffer and LayoutApplier. The new pass is integrated into the compilation pipelines to run after buffer flattening and before loop vectorization. Feedback on the changes suggests handling SBlockNode within the BoolBufferLegalizer to properly remap boolean buffers referenced or allocated inside blocks. Additionally, it is recommended to use new_buf->dtype instead of hardcoding DataType::Int(8) during load and store casts to improve code robustness.

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[tqchen]

actually i am not sure if bool to int8 lowering is still needed given we migrated into DLPack bool recently and now bool is native bool(8bit), cc @tlopex

[guan404ming]

Thanks @tqchen for catching this. Just confirmed it's dead code. With native DLPack bool(8-bit), LLVM lowers bool to byte-addressable i1 and CUDA emits native bool, so the int8 backing isn't needed. Verified end-to-end on LLVM CPU and CUDA L4: bool alloc buffers + load/store + compare/select all pass with the conversion removed. I have updated this PR to just delete the bool→int8 lowering instead of adding the pass.

[tqchen]

The name sounds it only works for Bool, instead, i think it is betetr to say BufferAccessType

and comment as the underlying access type for Buffer

[guan404ming]

Thanks for the great suggestion and I just updated.