Rewrite broadcast->layout

[pfultz2]

Motivation

Technical Details

Changelog Category

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• • Added: New functionality.
• • Changed: Changes to existing functionality.
• • Removed: Functionality or support that has been removed. (Compared to a previous release)
• • Optimized: Component performance that has been optimized or improved.
• • Resolved Issues: Known issues from a previous version that have been resolved.
• • Not Applicable: This PR is not to be included in the changelog.

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[Copilot]

Pull request overview

This PR updates the simplify_reshapes optimization pipeline to rewrite layout(broadcast(x)) / layout(multibroadcast(x)) into broadcast(layout(x)) / multibroadcast(layout(x)). This reduces unnecessary materialization by applying the layout transformation only to the unique (non-broadcasted) data, keeping broadcasted dimensions as broadcasted for downstream optimization and codegen.

Changes:

• Add a simplify_reshapes matcher (find_layout_broadcast) that pushes layout through broadcast/multibroadcast for static shapes.
• Compute an “inner” layout permutation that preserves the relative memory-ordering of the original input-aligned axes.
• Add unit tests covering multibroadcast and broadcast cases (including different broadcast axes) to validate the rewrite.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/simplify_reshapes.cpp	Adds a new simplify_reshapes rewrite to push layout before broadcast/multibroadcast to avoid full materialization of broadcasted outputs.
test/simplify_reshapes_test.cpp	Adds regression tests ensuring the new layout↔broadcast rewrite produces the expected transformed module.

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