[SPARK-6877][SQL] Add code generation support for Min

[viirya]

Currently min is not supported in code generation. This pr adds the support for it.

[SparkQA]

Test build #30153 has finished for PR 5487 at commit 0ddec23.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • class StringIndexer extends Estimator[StringIndexerModel] with StringIndexerBase
  • class VectorAssembler extends Transformer with HasInputCols with HasOutputCol
  • class VectorIndexer extends Estimator[VectorIndexerModel] with VectorIndexerParams
  • case class MinOf(left: Expression, right: Expression) extends Expression
• This patch does not change any dependencies.

[marmbrus]

Thanks, merged to master.