nir: Move new edgeflag assert into the io_lowered case

nir: Move new edgeflag assert into the io_lowered case

We only need to assert this in the `io_lowered` case, which actually
uses num_outputs.  This assert also doesn't appear to hold on iris,
where num_outputs is showing up as 0 (because it's likely not yet set).

Fixes assertion failures in edgeflag related tests on iris, which
doesn't use the io_lowered path currently.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3456
Fixes: 484a60d5474 ("nir: generate lowered IO in nir_lower_passthrough_edgeflags")
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6450>