For now these derefs are not handled, so don't let these get into the
copies list -- which would cause wrong propagations.  For load_derefs,
do nothing.  For store_derefs, invalidate whatever the store is
writing to.  For copy_derefs, invalidate whatever the copy is writing
to.

These cases will happen once derefs to SSBOs/UBOs are kept around long
enough to get optimized by copy_prop_vars.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>