Add reverse(reverse(x)) -> x optimization pattern

[Copilot]

Implements optimization to simplify consecutive stablehlo.reverse operations. When reversing the same dimension twice, the operations cancel out. This addresses chains of reverses appearing in autodiff-generated code (ref #1646).

Changes

• New ReverseReverse pattern in EnzymeHLOOpt.cpp: Computes symmetric difference of dimensions between consecutive reverses
  • Dimensions in both → cancel out (identity)
  • Dimensions in only one → remain
• Pattern registration in TransformOps.td as reverse_reverse
• Test coverage in reversereverse.mlir

Example

// Before
%0 = stablehlo.reverse %arg0, dims = [0, 1] : tensor<8x4x3xf32>
%1 = stablehlo.reverse %0, dims = [1, 2] : tensor<8x4x3xf32>

// After (dims [1] cancel, [0] and [2] remain)
%0 = stablehlo.reverse %arg0, dims = [0, 2] : tensor<8x4x3xf32>

When all dimensions cancel:

// Before
%0 = stablehlo.reverse %arg0, dims = [0] : tensor<8x4xf32>
%1 = stablehlo.reverse %0, dims = [0] : tensor<8x4xf32>

// After: no reverse ops, returns %arg0 directly

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Original prompt

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This section details on the original issue you should resolve

<issue_title>Reverse(reverse) -> noop</issue_title>
<issue_description>see #1646</issue_description>

<agent_instructions>Write an optimization in enzymehloopt.cpp like all the others in the file and corresponding utility files, that implements the optimization reverse(reverse(x)) -> x, where legal</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Reverse(reverse) -> noop #1648

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