PIG-3650: Fix for PIG-3100 breaks column pruning

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1556374 13f79535-47bb-0310-9956-ffa450edef68
PIG-3649: POPartialAgg incorrectly calculates size reduction when multiple values aggregated

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1556387 13f79535-47bb-0310-9956-ffa450edef68