Change overly strict Assert in TransactionGroupUpdateXidStatus.

This Assert thought that an overflowed transaction can never get registered
for the group update.  But that is not true, because even when the number
of children for a transaction got reduced, the overflow flag is not
changed.  And, for group update, we only care about the current number of
children for a transaction that is being committed.

Based on comments by Andres Freund, remove a redundant Assert in
TransactionIdSetPageStatus as we already had a static Assert for the same
condition a few lines earlier.

Reported-by: Vignesh C
Author: Dilip Kumar
Reviewed-by: Amit Kapila
Backpatch-through: 11
Discussion: https://postgr.es/m/CAFiTN-s5=uJw-Z6JC9gcqtBSjXsrHnU63PXBrA=pnBjqnkm5UA@mail.gmail.com

src/backend/access/transam/clog.c

@@ -299,13 +299,6 @@ TransactionIdSetPageStatus(TransactionId xid, int nsubxids,
 		memcmp(subxids, MyProc->subxids.xids,
 			   nsubxids * sizeof(TransactionId)) == 0)
 	{
-		/*
-		 * We don't try to do group update optimization if a process has
-		 * overflowed the subxids array in its PGPROC, since in that case we
-		 * don't have a complete list of XIDs for it.
-		 */
-		Assert(THRESHOLD_SUBTRANS_CLOG_OPT <= PGPROC_MAX_CACHED_SUBXIDS);
-
 		/*
 		 * If we can immediately acquire CLogControlLock, we update the status
 		 * of our own XID and release the lock.  If not, try use group XID
@@ -520,10 +513,10 @@ TransactionGroupUpdateXidStatus(TransactionId xid, XidStatus status,
 		PGXACT	   *pgxact = &ProcGlobal->allPgXact[nextidx];
 
 		/*
-		 * Overflowed transactions should not use group XID status update
-		 * mechanism.
+		 * Transactions with more than THRESHOLD_SUBTRANS_CLOG_OPT sub-XIDs
+		 * should not use group XID status update mechanism.
 		 */
-		Assert(!pgxact->overflowed);
+		Assert(pgxact->nxids <= THRESHOLD_SUBTRANS_CLOG_OPT);
 
 		TransactionIdSetPageStatusInternal(proc->clogGroupMemberXid,
 										   pgxact->nxids,