intel/compiler: Use nir_var_mem_task_payload

Instead of reusing the in/out slot mechanism, use a separated NIR
variable mode.  This will make easier later to implement staging the
output in shared memory (and storing all at the end to the URB).

Note to get 64-bit type support we currently rely on the
brw_nir_lower_mem_access_bit_sizes() pass.

Reviewed-by: Marcin Ślusarz <marcin.slusarz@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15022>

src/intel/compiler/brw_compiler.h

@@ -1412,8 +1412,6 @@ struct brw_clip_prog_data {
 };
 
 struct brw_tue_map {
-   int32_t start_dw[VARYING_SLOT_MAX];
-
    uint32_t size_dw;
 
    uint32_t per_task_data_start_dw;