[SPARK-20283][SQL] Add preOptimizationBatches

## What changes were proposed in this pull request?
We currently have postHocOptimizationBatches, but not preOptimizationBatches. This patch adds preOptimizationBatches so the optimizer debugging extensions are symmetric.

## How was this patch tested?
N/A

Author: Reynold Xin <rxin@databricks.com>

Closes #17595 from rxin/SPARK-20283.

sql/core/src/main/scala/org/apache/spark/sql/execution/SparkOptimizer.scala

@@ -30,13 +30,19 @@ class SparkOptimizer(
     experimentalMethods: ExperimentalMethods)
   extends Optimizer(catalog, conf) {
 
-  override def batches: Seq[Batch] = (super.batches :+
+  override def batches: Seq[Batch] = (preOptimizationBatches ++ super.batches :+
     Batch("Optimize Metadata Only Query", Once, OptimizeMetadataOnlyQuery(catalog, conf)) :+
     Batch("Extract Python UDF from Aggregate", Once, ExtractPythonUDFFromAggregate) :+
     Batch("Prune File Source Table Partitions", Once, PruneFileSourcePartitions)) ++
     postHocOptimizationBatches :+
     Batch("User Provided Optimizers", fixedPoint, experimentalMethods.extraOptimizations: _*)
 
+  /**
+   * Optimization batches that are executed before the regular optimization batches (also before
+   * the finish analysis batch).
+   */
+  def preOptimizationBatches: Seq[Batch] = Nil
+
   /**
    * Optimization batches that are executed after the regular optimization batches, but before the
    * batch executing the [[ExperimentalMethods]] optimizer rules. This hook can be used to add