nir_lower_readonly_images: Clear variable data when changing the type
For images, variable data includes the format. For samplers, variable data is used for OpenCL inline samplers. When converting a variable from one to the other, zero out the data so we don't accidentally interpret a converted image as an inline sampler.
Fixes: fa677c86 ("nir_lower_readonly_images_to_tex: Support non-CL semantics")