Fix DivMulFusion producer edge rewiring

[Sylvester Kaczmarek (sylvesterkaczmarek)]

Description

Fixes #32413, #32414 and #32416.

DivMulFusion rewrites 1 / x1 * x2 to x2 / x1 by replacing input 0 of the Div with the other Mul input. ReplaceNodeInput() only changes the NodeArg and intentionally does not create a graph edge.

When x2 is produced by another node, its producer edge still targets the Mul. FinalizeNodeFusion() then removes that edge with the Mul, leaving the new Div consuming a value without a producer edge. A later producer-elimination rule can remove the producer and leave an invalid graph.

This change adds the producer edge from the replacement input to the fused Div whenever the replacement input is node-produced. Graph inputs and initializers retain the existing behaviour.

Tests

Adds a regression graph with a node-produced Mul input and verifies the fused Div retains the producer connection and the graph resolves successfully.

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[Copilot]

🟢 Approval recommended

The focused fix correctly preserves graph connectivity and includes adequate regression coverage.

Pull request overview

Fixes DivMulFusion graph rewiring so node-produced replacement inputs retain their producer edges.

Changes:

• Adds the producer-to-fused-Div edge during rewiring.
• Adds regression coverage validating edge preservation and graph resolution.

File summaries

File	Description
onnxruntime/core/optimizer/div_mul_fusion.cc	Preserves producer edges during fusion.
onnxruntime/test/optimizer/div_mul_fusion_test.cc	Tests node-produced replacement inputs.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Balanced

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