[spirv-val] NV task shader should ignore 8/16 bit output restrictions 

taskNV payloads (input in mesh or output in task shader) can make use of 8/16 bit types independent of the capability that requires this for all shader inputs and outputs.

This wasn't properly spec'ed so far but it would be good if spirv-validation could stop complaining about this.

EXT was spec'ed differently and shouldn't be impacted.