nir: Add an IO scalarizing pass using the intrinsic's first_component.

nir: Add an IO scalarizing pass using the intrinsic's first_component.

vc4 wants to have per-scalar IO load/stores so that dead code elimination
can happen on a more granular basis, which it has been doing in the
backend using a multiplication by 4 of the intrinsic's driver_location.
We can represent it properly in the NIR using the first_component field,
though.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>