fix: place JOIN_RIGHT_SEMI at end of JoinType enum to preserve ABI

The initial backport (commit for aa86129e1) inserted JOIN_RIGHT_SEMI in
its upstream position, immediately before JOIN_RIGHT_ANTI.  In the
Greenplum/Cloudberry tree this shifts the integer values of the
GPDB-specific join codes that follow it -- JOIN_RIGHT_ANTI,
JOIN_UNIQUE_OUTER/INNER and especially JOIN_DEDUP_SEMI /
JOIN_DEDUP_SEMI_REVERSE.

Some value-dependent code relies on those stable integer values; shifting
them corrupts MPP motion planning.  Concretely, the regression query

  with ctetable as not materialized (select 1 as f1)
  select * from ctetable c1
  where f1 in (select c3.f1 from ctetable c2 full join ctetable c3 on true);

(which the baseline plans as a plain Hash Semi Join returning 1 row)
started producing a degenerate plan with "Gather Motion 0:1" /
"Redistribute Motion 1:0" and crashed with SIGSEGV in
setupCdbProcessList() during dispatch.

Isolation showed the crash reproduces with ONLY the enum change applied
(clean full rebuild) and disappears entirely once the pre-existing enum
values are kept stable.  Move JOIN_RIGHT_SEMI to the end of the enum so
every previously-defined value is unchanged.  JOIN_RIGHT_SEMI is still a
fully executor-supported join type; only its numeric position differs
from upstream.

Verified: the crashing query now returns 1 row under both optimizers, and
Hash Right Semi Join is still chosen (and correct) for the small-build-side
case.

Author: kongfanshen

src/include/nodes/nodes.h

@@ -1026,7 +1026,6 @@ typedef enum JoinType
 	JOIN_LASJ_NOTIN,			/* Left Anti Semi Join with Not-In semantics:
 									If any NULL values are produced by inner side,
 									return no join results. Otherwise, same as LASJ */
-	JOIN_RIGHT_SEMI,			/* 1 copy of each RHS row that has match(es) */
 	JOIN_RIGHT_ANTI,			/* 1 copy of each RHS row that has no match */
 
 	/*
@@ -1046,7 +1045,19 @@ typedef enum JoinType
 	 * moving the larger of the two relations.
 	 */
 	JOIN_DEDUP_SEMI,			/* inner join, LHS path must be made unique afterwards */
-	JOIN_DEDUP_SEMI_REVERSE		/* inner join, RHS path must be made unique afterwards */
+	JOIN_DEDUP_SEMI_REVERSE,	/* inner join, RHS path must be made unique afterwards */
+
+	/*
+	 * JOIN_RIGHT_SEMI (backported from upstream commit aa86129e1) is an
+	 * executor-supported join type, but it is deliberately placed at the END
+	 * of this enum rather than next to JOIN_RIGHT_ANTI where upstream puts it.
+	 * Inserting it in the middle would shift the integer values of the
+	 * GPDB-specific JOIN_DEDUP_SEMI/REVERSE (and JOIN_UNIQUE_*) codes, which
+	 * breaks value-dependent code elsewhere and corrupts MPP motion planning
+	 * (observed as a SIGSEGV during dispatch on semijoins over non-partitioned
+	 * loci).  Appending here keeps every pre-existing value stable.
+	 */
+	JOIN_RIGHT_SEMI				/* 1 copy of each RHS row that has match(es) */
 
 	/*
 	 * We might need additional join types someday.