nir: Add fsin_agx opcode

Used to split up the fsin/fcos lowering for AGX between NIR and the
backend, to permit algebraic optimizations without polluting NIR with
too many hardware details. The backend NIR lowering produces an
fmul/ffma of the input so we can optimize code like sin(2*x).

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10582>