Revert "Do execGrouping.c via expression eval machinery."

This reverts commit 773aec7aa9.

There's an unresolved issue in the reverted commit: It only creates
one comparator function, but in for the nodeSubplan.c case we need
more (c.f. FindTupleHashEntry vs LookupTupleHashEntry calls in
nodeSubplan.c).

This isn't too difficult to fix, but it's not entirely trivial
either. The fact that the issue only causes breakage on 32bit systems
shows that the current test coverage isn't that great.  To avoid
turning half the buildfarm red till those two issues are addressed,
revert.

src/include/executor/nodeAgg.h

@@ -102,12 +102,11 @@ typedef struct AggStatePerTransData
 	bool	   *sortNullsFirst;
 
 	/*
-	 * Comparators for input columns --- only set/used when aggregate has
-	 * DISTINCT flag. equalfnOne version is used for single-column
-	 * commparisons, equalfnMulti for the case of multiple columns.
+	 * fmgr lookup data for input columns' equality operators --- only
+	 * set/used when aggregate has DISTINCT flag.  Note that these are in
+	 * order of sort column index, not parameter index.
 	 */
-	FmgrInfo	equalfnOne;
-	ExprState  *equalfnMulti;
+	FmgrInfo   *equalfns;		/* array of length numDistinctCols */
 
 	/*
 	 * initial value from pg_aggregate entry
@@ -271,8 +270,7 @@ typedef struct AggStatePerPhaseData
 	int			numsets;		/* number of grouping sets (or 0) */
 	int		   *gset_lengths;	/* lengths of grouping sets */
 	Bitmapset **grouped_cols;	/* column groupings for rollup */
-	ExprState **eqfunctions;	/* expression returning equality, indexed by
-								 * nr of cols to compare */
+	FmgrInfo   *eqfunctions;	/* per-grouping-field equality fns */
 	Agg		   *aggnode;		/* Agg node for phase data */
 	Sort	   *sortnode;		/* Sort node for input ordering for phase */