pgsql: Factor out constructSetOpTargetlist() from transformSetOperation

Factor out constructSetOpTargetlist() from transformSetOperationTree()

This would be used separately by a future patch.  It also makes a
little smaller.

Author: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/a855795d-e697-4fa5-8698-d20122126567@eisentraut.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8c2b30487cc7e8797ca78f3b089ae0fec199f1a9

Modified Files
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src/backend/parser/analyze.c | 313 +++++++++++++++++++++++--------------------
1 file changed, 168 insertions(+), 145 deletions(-)