HIVE-8313: Optimize evaluation for ExprNodeConstantEvaluator and ExprNodeNullEvaluator (Mithun Radhakrishnan, reviewed by Navis)

git-svn-id: https://svn.apache.org/repos/asf/hive/trunk@1636227 13f79535-47bb-0310-9956-ffa450edef68