agx: Don't set lower_pack_split

agx: Don't set lower_pack_split

We should handle nir_op_unpack_32_2x16_split_* natively, since we can generate
better code with agx_subdivide (coalescing the ops away) than the bitshift
lowering.

That said, we do need some extra instructions for the floating point
conversions.

No shader-db changes (which makes sense because we're targetting the GLES3.0
shader-db, which doesn't have the packing GLSL functions).

The real motivation of this change isn't optimizing some GLSL pack functions,
though, it's avoiding a code regression from using NIR's memory bit size
lowering in a future MR. That lowering will turn things like "load i16vec4" into
"load i32vec2 + unpack_32_2x16", so we need to be able to coalesce that unpack.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21674>