Fix estimate_expression_value to constant-fold SQLValueFunction nodes.
Fix estimate_expression_value to constant-fold SQLValueFunction nodes.

Oversight in my commit 0bb51aa96.  Noted while poking at a recent
bug report --- HEAD's estimates for a query using CURRENT_DATE
were unexpectedly much worse than 9.6's.

lisakowen/gpdb,adam8157/gpdb,xinzweb/gpdb,50wu/gpdb,greenplum-db/gpdb,50wu/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,greenplum-db/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,50wu/gpdb,lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb,50wu/gpdb,xinzweb/gpdb,adam8157/gpdb,lisakowen/gpdb,adam8157/gpdb,xinzweb/gpdb,lisakowen/gpdb,50wu/gpdb,adam8157/gpdb,lisakowen/gpdb,greenplum-db/gpdb,adam8157/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,xinzweb/gpdb,50wu/gpdb,adam8157/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,adam8157/gpdb,xinzweb/gpdb,adam8157/gpdb