spirv, nir: Handle EmitMeshTasksEXT opcode.

spirv, nir: Handle EmitMeshTasksEXT opcode.

A task shader must use this instruction to specify the dimensions
of the launched mesh shader workgroups.
It is a terminating instruction.

When the task shader doesn't have the optional payload, use the
pre-existing launch_mesh_workgroups intrinsics.

When the task shader has a payload, use a new
launch_mesh_workgroups_with_payload_deref intrinsics which has
a deref that refers to the payload variable.

We also add this new intrinsic to nir_lower_io which lowers this
to the pre-existing explicit intrinsic.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18366>