Set indcheckxmin true when REINDEX fixes an invalid or not-ready index.

Per comment from Greg Stark, it's less clear that HOT chains don't conflict
with the index than it would be for a valid index.  So, let's preserve the
former behavior that indcheckxmin does get set when there are
potentially-broken HOT chains in this case.  This change does not cause any
pg_index update that wouldn't have happened anyway, so we're not
re-introducing the previous bug with pg_index updates, and surely the case
is not significant from a performance standpoint; so let's be as
conservative as possible.

src/backend/catalog/index.c

@@ -2800,7 +2800,12 @@ reindex_index(Oid indexId, bool skip_constraint_checks)
 	 *
 	 * We can also reset indcheckxmin, because we have now done a
 	 * non-concurrent index build, *except* in the case where index_build
-	 * found some still-broken HOT chains.
+	 * found some still-broken HOT chains.  If it did, we normally leave
+	 * indcheckxmin alone (note that index_build won't have changed it,
+	 * because this is a reindex).  But if the index was invalid or not ready
+	 * and there were broken HOT chains, it seems best to force indcheckxmin
+	 * true, because the normal argument that the HOT chains couldn't conflict
+	 * with the index is suspect for an invalid index.
 	 *
 	 * Note that it is important to not update the pg_index entry if we don't
 	 * have to, because updating it will move the index's usability horizon
@@ -2825,10 +2830,12 @@ reindex_index(Oid indexId, bool skip_constraint_checks)
 		if (!indexForm->indisvalid || !indexForm->indisready ||
 			(indexForm->indcheckxmin && !indexInfo->ii_BrokenHotChain))
 		{
-			indexForm->indisvalid = true;
-			indexForm->indisready = true;
 			if (!indexInfo->ii_BrokenHotChain)
 				indexForm->indcheckxmin = false;
+			else if (!indexForm->indisvalid || !indexForm->indisready)
+				indexForm->indcheckxmin = true;
+			indexForm->indisvalid = true;
+			indexForm->indisready = true;
 			simple_heap_update(pg_index, &indexTuple->t_self, indexTuple);
 			CatalogUpdateIndexes(pg_index, indexTuple);
 		}