[SPARK-16640][SQL] Add codegen for Elt function

## What changes were proposed in this pull request?

Elt function doesn't support codegen execution now. We should add the support.

## How was this patch tested?

Jenkins tests.

Author: Liang-Chi Hsieh <simonh@tw.ibm.com>

Closes #14277 from viirya/elt-codegen.