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>

src/compiler/nir/nir.h

@@ -2395,6 +2395,7 @@ void nir_lower_alu_to_scalar(nir_shader *shader);
 void nir_lower_load_const_to_scalar(nir_shader *shader);
 
 void nir_lower_phis_to_scalar(nir_shader *shader);
+void nir_lower_io_to_scalar(nir_shader *shader, nir_variable_mode mask);
 
 void nir_lower_samplers(nir_shader *shader,
                         const struct gl_shader_program *shader_program);