pan/mdg: Add disassembly for shadow gathers

pan/mdg: Add disassembly for shadow gathers

By applying `textureGather` to a `sampler2DShadow`, the blob produces
(under the old disassembly):

   tex_22.vtx.2d.shadow.cont.last r29, texture0, fsampler0.zwyx, r29,

The op 0x22 is 10|0010 in binary, the old shadow parameter is 1, and old
gather parameter is 0, so we get 0110|0010 in binary, or an op of
textureLod with a mod of 0110 = 6.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6516>