pgsql: Use outerPlanState macro instead of referring to leffttree.

Use outerPlanState macro instead of referring to leffttree.

This makes the executor code more consistent.  It also removes
an apparently superfluous NULL test in nodeGroup.c.

Qingqing Zhou, reviewed by Tom Lane, and further revised by me.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/40f42d2a34329b0b71a1287d6fd2554298dbb713

Modified Files
--------------
src/backend/executor/nodeAgg.c            |    7 ++++---
src/backend/executor/nodeBitmapHeapscan.c |    6 ++++--
src/backend/executor/nodeGroup.c          |    7 ++++---
src/backend/executor/nodeMaterial.c       |   12 +++++++-----
src/backend/executor/nodeSort.c           |    8 +++++---
src/backend/executor/nodeWindowAgg.c      |    5 +++--
6 files changed, 27 insertions(+), 18 deletions(-)