Lower branched A16W8 convs on U55 (#22566)

[apullin]

Summary:

A16W8 convolution rewriting can expose one INT48 accumulator to multiple output RESCALE nodes. Regor cannot materialize INT48 on U55 and can only fuse one output quantization into each convolution, so these graphs fail compilation before scheduling.

This fixes a U55 lowering regression introduced by #21910: its A16W8 output branching can leave one INT48 convolution accumulator feeding multiple RESCALE paths, but Regor cannot materialize INT48 and can fold only one RESCALE into a convolution.

Deduplicate identical complete RESCALE plus layout-permute heads, then clone the TOSA convolution for each remaining distinct output quantization while sharing its input, weight, and bias nodes. Mark those exact-semantics clones so later duplicate-user cleanup cannot merge the unsupported fanout back together.

Add structural, duplicate-fusion barrier, and true U55 compilation regressions. This change was authored with Codex.

Differential Revision: D118736607

[pytorch-bot]

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❌ 3 New Failures, 1 Pending, 1 Unrelated Failure

As of commit 193e773 with merge base 4ce2ec2 ():

NEW FAILURES - The following jobs have failed:

• Lint / lintrunner (gh)
  >>> Lint for backends/arm/_passes/rewrite_conv_pass.py:
• Lint / lintrunner-mypy (gh)
  >>> Lint for backends/arm/_passes/rewrite_conv_pass.py:
• trunk / unittest-release / macos / macos-job (gh)
  executorch

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• trunk / test-models-macos-coreml (vit) / macos-job (gh) (matched macos rule in flaky-rules.json)
  ##[error]File doesn't exist

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[meta-codesync]

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[sylvesterkaczmarek]

The repair matches the U55 constraint: deduplicate truly identical RESCALE→PERMUTE heads first, then clone the INT48-producing convolution only for semantically distinct quantisation heads. Marking the clones against later duplicate-user fusion is important, otherwise a later cleanup can recreate the unsupported fanout. The signature and unknown-user guards keep the rewrite conservative.

[apullin]

Local lint stepped on a bunch of comments and unrelated code. Should be cleaned up now, for just the relevant changes.