radv: Allow using the high 16 bits of PS inputs.
Some minor refactoring to make the code a bit more readable.
Add a new float16_hi_shaded_mask to keep track of which PS input slots use their high 16 bits, based on the high_16bits of the NIR IO semantics. Then, set ATTR1_VALID accordingly.
This will enable RADV to handle the case when the two 16-bit PS inputs are packed into one slot.
Note that ACO already handles this case, but the RADV side was completely missing.