Don't call IS_DUMMY_REL() when cheapest_total_path might be junk.

Don't call IS_DUMMY_REL() when cheapest_total_path might be junk.

Unlike the previous coding, this might result in a Gather per Append
subplan when the target list is parallel-restricted, but such a plan
is probably worth considering in that case, since a single Gather
on top of the entire Append is impossible.

Per Andres Freund and the buildfarm.

Discussion: http://postgr.es/m/20180330050351.bmxx4cdtz67czjda@alap3.anarazel.de