Use macro NUM_MERGE_MATCH_KINDS instead of '3' in MERGE code.

Code quality improvement for 0294df2f1f.

Aleksander Alekseev, reviewed by Richard Guo.

Discussion: https://postgr.es/m/CAJ7c6TMsiaV5urU_Pq6zJ2tXPDwk69-NKVh4AMN5XrRiM7N%2BGA%40mail.gmail.com

src/backend/optimizer/prep/prepjointree.c

@@ -153,7 +153,7 @@ transform_MERGE_to_join(Query *parse)
 {
 	RangeTblEntry *joinrte;
 	JoinExpr   *joinexpr;
-	bool		have_action[3];
+	bool		have_action[NUM_MERGE_MATCH_KINDS];
 	JoinType	jointype;
 	int			joinrti;
 	List	   *vars;

src/backend/parser/parse_merge.c

@@ -109,7 +109,7 @@ transformMergeStmt(ParseState *pstate, MergeStmt *stmt)
 	Query	   *qry = makeNode(Query);
 	ListCell   *l;
 	AclMode		targetPerms = ACL_NO_RIGHTS;
-	bool		is_terminal[3];
+	bool		is_terminal[NUM_MERGE_MATCH_KINDS];
 	Index		sourceRTI;
 	List	   *mergeActionList;
 	ParseNamespaceItem *nsitem;

src/include/nodes/execnodes.h

@@ -545,7 +545,7 @@ typedef struct ResultRelInfo
 	OnConflictSetState *ri_onConflict;
 
 	/* for MERGE, lists of MergeActionState (one per MergeMatchKind) */
-	List	   *ri_MergeActions[3];
+	List	   *ri_MergeActions[NUM_MERGE_MATCH_KINDS];
 
 	/* for MERGE, expr state for checking the join condition */
 	ExprState  *ri_MergeJoinCondition;

src/include/nodes/primnodes.h

@@ -1970,6 +1970,8 @@ typedef enum MergeMatchKind
 	MERGE_WHEN_NOT_MATCHED_BY_TARGET
 } MergeMatchKind;
 
+#define NUM_MERGE_MATCH_KINDS (MERGE_WHEN_NOT_MATCHED_BY_TARGET + 1)
+
 typedef struct MergeAction
 {
 	NodeTag		type;