[SPARK-12978][SQL] Skip unnecessary final group-by when input data already clustered with group-by keys
This ticket targets the optimization to skip an unnecessary group-by operation below;
Without opt.:
```
== Physical Plan ==
TungstenAggregate(key=[col0#159], functions=[(sum(col1#160),mode=Final,isDistinct=false),(avg(col2#161),mode=Final,isDistinct=false)], output=[col0#159,sum(col1)#177,avg(col2)#178])
+- TungstenAggregate(key=[col0#159], functions=[(sum(col1#160),mode=Partial,isDistinct=false),(avg(col2#161),mode=Partial,isDistinct=false)], output=[col0#159,sum#200,sum#201,count#202L])
+- TungstenExchange hashpartitioning(col0#159,200), None
+- InMemoryColumnarTableScan [col0#159,col1#160,col2#161], InMemoryRelation [col0#159,col1#160,col2#161], true, 10000, StorageLevel(true, true, false, true, 1), ConvertToUnsafe, None
```
With opt.:
```
== Physical Plan ==
TungstenAggregate(key=[col0#159], functions=[(sum(col1#160),mode=Complete,isDistinct=false),(avg(col2#161),mode=Final,isDistinct=false)], output=[col0#159,sum(col1)#177,avg(col2)#178])
+- TungstenExchange hashpartitioning(col0#159,200), None
+- InMemoryColumnarTableScan [col0#159,col1#160,col2#161], InMemoryRelation [col0#159,col1#160,col2#161], true, 10000, StorageLevel(true, true, false, true, 1), ConvertToUnsafe, None
```
Author: Takeshi YAMAMURO <linguin.m.s@gmail.com>
Closes #10896 from maropu/SkipGroupbySpike.
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