pgsql: Fix ADD IF NOT EXISTS used in conjunction with ALTER TABLE ONLY

Fix ADD IF NOT EXISTS used in conjunction with ALTER TABLE ONLY

The flag for IF NOT EXISTS was only being passed down in the normal
recursing case. It's been this way since originally added in 9.6 in
commit 2cd40adb85 so backpatch back to 9.6.

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/5668afeb6d25ef842ff14239c5ba3aa20cf56937

Modified Files
--------------
src/backend/commands/tablecmds.c          | 2 +-
src/test/regress/expected/alter_table.out | 6 ++++++
src/test/regress/sql/alter_table.sql      | 4 ++++
3 files changed, 11 insertions(+), 1 deletion(-)