nir: Make nir_rewrite_image_intrinsic() robust to invalid src/dst_type
What does this MR do and why?
Mimic what's done in lower_io() and infer the src/dst_type of the image
intrinsic from the variable sampled_type when src/dst_type is invalid.
This avoids extra churn when emitting deref-based image operations where
the sampled type is already defined at the variable level.
Was needed for !26739, and I also hit the issue recently when working on transition to vk_meta in panvk. Not sure if this is the right fix though.
/cc @rsmith