pgsql: Fix IF NOT EXISTS in CREATE STATISTICS

Fix IF NOT EXISTS in CREATE STATISTICS

I misplaced the IF NOT EXISTS clause in commit 7b504eb282, before the
word STATISTICS. Put it where it belongs.

Patch written independently by Amit Langote and myself. I adopted his
submitted test case with a slight edit also.

Reported-by: Bruno Wolff III
Discussion: https://postgr.es/m/20170621004237.GB8337@wolff.to

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5dfd564b1001f20c9def6ae938a92f39ddbd9984

Modified Files
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src/backend/parser/gram.y | 25 ++++++++++++++++++-------
src/test/regress/expected/stats_ext.out | 6 ++++--
src/test/regress/sql/stats_ext.sql | 5 +++--
3 files changed, 25 insertions(+), 11 deletions(-)