hexagon: eliminate scalar VTCM loads via HVX splat helpers (#22993)

* hexagon: add hvx_vec_repl helpers and use those for splat-from-vtcm usecase

* hmx-mm: optimize per-group scale handling

* hmx-fa: optimize slope load from vtcm

* hmx-fa: use aligned access where possible in hmx-utils

* hexagon: add hvx_vec_repl_2x_f16 helper and consolidate repl helpers

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Co-authored-by: Max Krasnyansky <maxk@qti.qualcomm.com>