pgsql: Avoid creating PlaceHolderVars immediately within PlaceHolderVar

Avoid creating PlaceHolderVars immediately within PlaceHolderVars.

Such a construction is useless since the lower PlaceHolderVar is already
nullable; no need to make it more so.  Noted while pursuing bug #6154.

This is just a minor planner efficiency improvement, since the final plan
will come out the same anyway after PHVs are flattened.  So not worth the
risk of back-patching.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/cff60f2dfa470d5736a19d36eb910844e31db764

Modified Files
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src/backend/optimizer/prep/prepjointree.c |   20 ++++++++++++++++----
1 files changed, 16 insertions(+), 4 deletions(-)