iris: Rework edgeflag handling

iris: Rework edgeflag handling

We were relying on specific pass ordering in st to avoid setting
inputs_read/outputs_written for edge flags.  Instead, just assume
that it happens and throw out the results we don't want.

We should probably revisit this and try and add a vertex element
property like I originally wanted so we can avoid having it be
associated with the VS altogether.