private SemanticNodeProcessor getValueAliasProcessor() {
    return new ValueAliasProcessor();
  }
Function to do groupby subexpression elimination. This is called by all the
processors initially. As an example, consider the query select a+b,
count(1) from T group by a+b; Then a+b is already precomputed in the group
by operators key, so we substitute a+b in the select list with the internal
column name of the a+b expression that appears in the in input row
resolver.

@param nd      The node that is being inspected.
@param procCtx The processor context.
@return exprNodeColumnDesc.
ql/src/java/org/apache/hadoop/hive/ql/parse/type/TypeCheckProcFactory.java