pgsql: Don't call IS_DUMMY_REL() when cheapest_total_path might bejunk

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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/96030f9a481a78483945447e122a387902a4549b

Modified Files
--------------
src/backend/optimizer/plan/planner.c | 32 +++++++++++++++++---------------
1 file changed, 17 insertions(+), 15 deletions(-)