[SPARK-17273][SQL] Move expression optimizer rules into a separate file

## What changes were proposed in this pull request?
As part of breaking Optimizer.scala apart, this patch moves various expression optimization rules into a single file.

## How was this patch tested?
This should be covered by existing tests.

Author: Reynold Xin <rxin@databricks.com>

Closes apache#14845 from rxin/SPARK-17273.

Author: rxin