Fix IndexError in RemovePermutesAroundElementwiseOps traversal

[cceroici]

Summary:
_adapt_permute_across_view indexed the permutation without checking it was expressed in the view's input-rank space, so visit() crashed with IndexError: list index out of range instead of rejecting the subgraph.

• Traversal adapts the permutation when it crosses a rank-changing view. If it then reaches a second squeeze/unsqueeze view whose input is still at the original rank, permute[index] is out of range — e.g. squeezing a trailing size-1 dim gives index == 3 for a rank-3 permutation.
• Fix: validate the index / rank in all three branches of _adapt_permute_across_view and return None, the function's documented "not possible" signal. All three call sites already handle None by skipping the subgraph.
• This mirrors the guard D105787161 added in permute_subgraph(), which only runs at apply time — traversal had no equivalent check.

Surfaced by D113424191: its _is_permutation_sink_view branch lets traversal continue past a layout-invariant flatten rather than aborting, which grows regions enough to reach a rank-mismatched view. That optimization is intentional and is preserved here; only the crash is fixed. The latent bug predates it.

Broke test_auth_u85 (ECAPA -> Ethos-U85) — ARM's RemovePermutesAroundElementwiseTosaOps subclasses this shared pass, so it runs inside ArmPassManager on every Ethos-U lowering. That test is CONTINUOUS_STATE, so it is omitted on diffs and could not have blocked the landing.

Authored with AI assistance (Claude Code).

Differential Revision: D115062769

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[meta-codesync]

@cceroici has exported this pull request. If you are a Meta employee, you can view the originating Diff in D115062769.