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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master

Details
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https://git.postgresql.org/pg/commitdiff/1075dfdaf33ad8b2a61c26b5748735005e6192b9

Modified Files
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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(-)