This allows mediump inputs and outputs to be trivially lowered into packed
16-bit varyings where 1 slot is occupied by 2 16-bit vec4s, without any
packing instructions in NIR and without any conflicts with 32-bit varyings.
The only thing that is changed is IO semantics in intrinsics to get packed
16-bit varyings.
This simplifies supporting 16-bit types for drivers that have 32-bit slots
everywhere except the fragment shader where they can do 16-bit interpolation
on either the low or high half of each slot.
Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9050>
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