nir: Allow using nir_lower_io_to_scalar_early on VS input vars.

This will be used on V3D to cut down the size of the VS inputs in the VPM
(memory area for sharing data between shader stages).

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>

src/compiler/nir/nir_lower_io_to_scalar.c

@@ -320,7 +320,9 @@ nir_lower_io_to_scalar_early(nir_shader *shader, nir_variable_mode mask)
                if (glsl_type_is_64bit(glsl_without_array(var->type)))
                   continue;
 
-               if (var->data.location < VARYING_SLOT_VAR0 &&
+               if (!(shader->info.stage == MESA_SHADER_VERTEX &&
+                     mode == nir_var_shader_in) &&
+                   var->data.location < VARYING_SLOT_VAR0 &&
                    var->data.location >= 0)
                   continue;