Fix IndexStmt comparison function

The altconname member defines an alternative name for the index if
another name for a constraint was requested. Including it in the
comparison is mostly for correctness, as the index name is compared,
even if few (if any) situations would fail on this being omitted.
Reviewed-by: NJacob Champion <pchampion@pivotal.io>
Reviewed-by: NVenkatesh Raghavan <vraghavan@pivotal.io>

src/backend/nodes/equalfuncs.c

@@ -1471,7 +1471,7 @@ _equalIndexStmt(IndexStmt *a, IndexStmt *b)
 	COMPARE_SCALAR_FIELD(deferrable);
 	COMPARE_SCALAR_FIELD(initdeferred);
 	COMPARE_SCALAR_FIELD(concurrent);
-	/* GPDB_90_MERGE_FIXME: should we compare altconname? */
+	COMPARE_STRING_FIELD(altconname);
 	COMPARE_SCALAR_FIELD(is_split_part);
 
 	return true;