[Unity][Fix] Annotate TIR op pattern could have no stores.

[psrivas2]

Fix a bug where a block could have no stores, the pass crashes in such a case. We should mark the PrimFunc as opaque in such a case.
This happens if instead of using buffer load/stores we use tir.call_packed for computation.

cc @jwfromm @sunggg

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• cc @quic-sanirudh _{See #10317 for details}

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[sunggg]

Thanks for the fix! It would be nice to add more context about when we face such cases in the comment.

[psrivas2]

Thanks for the fix! It would be nice to add more context about when we face such cases in the comment.

added a test and some context.

[sunggg]

@psrivas2 Thank you!