Simplify representation of aggregate transition values a bit.

Previously aggregate transition values for hash and other forms of
aggregation (i.e. sort and no group by) were represented
differently. Hash based aggregation used a grouping set indexed array
pointing to an array of transition values, whereas other forms of
aggregation used one flattened array with the index being computed out
of grouping set and transition offsets.

That made upcoming changes hard, so represent both as grouping set
indexed array of per-group data.

As a nice side-effect this also makes aggregation slightly faster,
because computing offsets with `transno + (setno * numTrans)` turns
out not to be that cheap (too big for x86 lea for example).

Author: Andres Freund
Discussion: https://postgr.es/m/20171128003121.nmxbm2ounxzb6n2t@alap3.anarazel.de

src/include/nodes/execnodes.h

@@ -1852,13 +1852,15 @@ typedef struct AggState
 	Tuplesortstate *sort_out;	/* input is copied here for next phase */
 	TupleTableSlot *sort_slot;	/* slot for sort results */
 	/* these fields are used in AGG_PLAIN and AGG_SORTED modes: */
-	AggStatePerGroup pergroup;	/* per-Aggref-per-group working state */
+	AggStatePerGroup *pergroups;	/* grouping set indexed array of per-group
+									 * pointers */
 	HeapTuple	grp_firstTuple; /* copy of first tuple of current group */
 	/* these fields are used in AGG_HASHED and AGG_MIXED modes: */
 	bool		table_filled;	/* hash table filled yet? */
 	int			num_hashes;
 	AggStatePerHash perhash;
-	AggStatePerGroup *hash_pergroup;	/* array of per-group pointers */
+	AggStatePerGroup *hash_pergroup;	/* grouping set indexed array of
+										 * per-group pointers */
 	/* support for evaluation of agg input expressions: */
 	ProjectionInfo *combinedproj;	/* projection machinery */
 } AggState;