pgsql: Fix MERGE's test for unreachable WHEN clauses.

Fix MERGE's test for unreachable WHEN clauses.

The former code would only detect an unreachable WHEN clause if it had
an AND condition. Fix, so that unreachable unconditional WHEN clauses
are also detected.

Back-patch to v15, where MERGE was added.

Discussion: https://postgr.es/m/CAEZATCVQ=7E2z4cSBB49jjeGGsB6WeoYQY32NDeSvcHiLUZ=ow@mail.gmail.com

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/38255f2d0059a101c3fb791d6523a9e66e55aa66

Modified Files
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src/backend/parser/parse_merge.c    | 6 +++---
src/test/regress/expected/merge.out | 2 +-
src/test/regress/sql/merge.sql      | 2 +-
3 files changed, 5 insertions(+), 5 deletions(-)