[SPARK-23389][CORE] When the shuffle dependency specifies aggregation ,and...

[SPARK-23389][CORE] When the shuffle dependency specifies aggregation ,and `dependency.mapSideCombine =false`, we should be able to use serialized sorting.

## What changes were proposed in this pull request?
When the shuffle dependency specifies aggregation ,and `dependency.mapSideCombine=false`, in the map side,there is no need for aggregation and sorting, so we should be able to use serialized sorting.

## How was this patch tested?
Existing unit test

Author: liuxian <liu.xian3@zte.com.cn>

Closes #20576 from 10110346/mapsidecombine.