[SPARK-18711][SQL] should disable subexpression elimination for LambdaVariable

## What changes were proposed in this pull request?

This is kind of a long-standing bug, it's hidden until https://github.com/apache/spark/pull/15780 , which may add `AssertNotNull` on top of `LambdaVariable` and thus enables subexpression elimination.

However, subexpression elimination will evaluate the common expressions at the beginning, which is invalid for `LambdaVariable`. `LambdaVariable` usually represents loop variable, which can't be evaluated ahead of the loop.

This PR skips expressions containing `LambdaVariable` when doing subexpression elimination.

## How was this patch tested?

updated test in `DatasetAggregatorSuite`

Author: Wenchen Fan <wenchen@databricks.com>

Closes #16143 from cloud-fan/aggregator.