Remove redundancy in run-time generated code for common column references.

When a operator (Filter, project) has expression which refer one same field multiple times, Drill will initialize a value vector and do value holder assignment for each field reference in the run-time generated code. The redundancy might impact the expression evaluation, after the compiled code is executed over large number of incoming rows.

This has been seen in recent performance issue reported on the drill user list, where the query contains multiple multiple in list filter conditions.

In this JIRA, we'll remove the redundancy for the common field reference, so that only one initialization and assignment happen in the run-time generated code.