pgsql: Reorder tests in get_cheapest_path_for_pathkeys().

Reorder tests in get_cheapest_path_for_pathkeys().

Checking parallel safety should be even cheaper than cost comparison, so
do that first.

Also make some minor, related comment improvements.

Richard Guo, reviewed by Aleksander Alekseev, Andy Fan, and me.

Discussion: http://postgr.es/m/CAMbWs4-KE2wf4QPj_Sr5mX4QFtBNNKGmxK=+e=KZEGUjdG33=g@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9caf042088e7416ed612e52519ee15f0717e86a7

Modified Files
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src/backend/optimizer/path/pathkeys.c | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)