[TIR] Update symbolic index term order in loop fusion

[wrongtest-intellif]

This change just keep stride terms order the same with fused loop order in fuse primitive. In symbolic circumstances, previous form suffer from simplification issues and would make the expression tree much complex in following lowering steps.

Take [M, N] tiling as an example, the previous binding form after

i, j = sch.get_loops(block_b)
i0, i1 = sch.split(i, factors=[None, 64])
j0, j1 = sch.split(j, factors=[None, 16])
sch.reorder(i0, j0, i1, j1)
sch.fuse(i0, j0)

would be like (i_0_j_0_fused in [0, ceildiv(M, 64) * ceildiv(N, 16)]

vi = T.axis.spatial(M, i_0_j_0_fused % ((N + 15) // 16 * ((M + 63) // 64)) // ((N + 15) // 16) * 64 + i_1)

instead of more simple version

vi = T.axis.spatial(M, i_0_j_0_fused // ((N + 15) // 16) * 64 + i_1)

This is because unfortunately we do not know ceildiv(N, 16) * ceildiv(M, 64) == ceildiv(M, 64) * ceildiv(N, 16) in rule based simplifications. And then certain analysis (for example, region estimation) may fail to give concise estimations, due to complex dynamic expression trees.

[gemini-code-assist]

Summary of Changes

Hello @wrongtest-intellif, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refines the symbolic index term ordering within the loop fusion mechanism of the TIR schedule. The adjustment aims to simplify generated expressions, particularly in symbolic contexts, which previously led to complex expression trees and hindered analysis like region estimation. By ensuring that stride terms maintain an order consistent with the fused loops, the change improves the efficiency and clarity of the intermediate representation, making subsequent lowering steps more robust.

Highlights

• Symbolic Index Term Order: The pull request updates the order of symbolic index terms within the loop fusion primitive to ensure consistency with the fused loop order.
• Expression Simplification: This change addresses previous issues where symbolic expressions became overly complex, hindering simplification and analysis (e.g., region estimation).
• Code Logic Adjustment: The core logic in loop_transformation.cc was modified to reorder multiplication terms in floormod and lower updates, and the fused_extent calculation loop direction was reversed.
• New Test Case: A new test, test_fused_symbolic_2D_tiling, has been added to validate the improved symbolic index term ordering after loop fusion with 2D tiling.

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[gemini-code-assist]

Code Review

This pull request addresses a symbolic simplification issue within the fuse primitive in TIR. The changes modify the order of multiplication for calculating the fused loop extent and the strides for substituted loop variables. By making the order of terms consistent with the fused loop order, the resulting expression trees for loop variable bindings become simpler, which aids downstream analysis and simplification passes, especially with symbolic shapes. A new test case is added to verify this fix for a 2D tiling scenario, confirming that the generated code is now simpler as expected. The changes are logical, well-contained, and effectively solve the described problem.