[Arith] Simplify to positive numerators in floordiv/floormod

[Lunderberg]

Negative numerators to modulo/remainder operations are not supported by the Vulkan API. While the SPIR-V instructions
OpSRem and OpSMod have identical semantics to tir::Mod and tir::FloorMod, respectively, use of either instruction within Vulkan results in undefined behavior. From the Vulkan spec:

For the OpSRem and OpSMod instructions, if either operand is negative the result is undefined.

Note: While the OpSRem and OpSMod instructions are supported by the Vulkan environment, they require non-negative values and thus do not enable additional functionality beyond what OpUMod provides.

This issue was first noticed in #13530, where use of integer arithmetic resulted in negative numerators. This hadn't caused issues previously, because most use of div/mod use a denominator that is a power of two. In these cases, tir.LowerIntrin implements floordiv and floormod using only bitwise operations. When the denominator isn't a power of two, both tir::FloorDiv and tir::FloorMod are implemented in terms of tir::Mod, which triggers the undefined behavior for negative numerators.

This commit implements additional simplification rules that preferentially removes negative values from the numerators. For example, simplifying floormod(i - 2, 8) to floormod(i + 6, 8), and simplifying floordiv(i - 2, 8) to floordiv(i + 6, 8) - 1. These handle the most common case, where some index variable is being offset by a negative constant.

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• cc @junrushao _{See #10317 for details}

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[Lunderberg]

When implementing this change, I had also considered making a Vulkan-specific pass, rather than changing the simplification. However, the effect of such a pass could be undone by a later simplification, so I believe this to be the better solution.

[Lunderberg]

The latest change to the simplification caused more breakages than I think it is worth. Closing this PR in favor of #13724, which has a more limited scope.