Add a macro to provide universal consistency around the check

for what sort of creatures tend to revive.

include/mondata.h

@@ -129,6 +129,9 @@
 				 (ptr) == &mons[PM_GIANT] || \
 				 (ptr) == &mons[PM_ELF] || \
 				 (ptr) == &mons[PM_HUMAN])
+/* return TRUE if the monster tends to revive */
+#define is_reviver(ptr)		(is_rider(ptr) || (ptr)->mlet == S_TROLL)
+
 /* this returns the light's range, or 0 if none; if we add more light emitting
    monsters, we'll likely have to add a new light range field to mons[] */
 #define emits_light(ptr)	(((ptr)->mlet == S_LIGHT || \

src/invent.c

@@ -2155,7 +2155,7 @@ mergable(otmp, obj)	/* returns TRUE if obj  & otmp can be merged */
 	/* hatching eggs don't merge; ditto for revivable corpses */
 	if ((obj->otyp == EGG && (obj->timed || otmp->timed)) ||
 	    (obj->otyp == CORPSE && otmp->corpsenm >= LOW_PM &&
-		mons[otmp->corpsenm].mlet == S_TROLL))
+		is_reviver(&mons[otmp->corpsenm])))
 	    return FALSE;
 
 	/* allow candle merging only if their ages are close */

src/mon.c

@@ -100,11 +100,8 @@ STATIC_VAR short cham_to_pm[] = {
 		PM_SANDESTIN,
 };
 
-/* return TRUE if the monster tends to revive */
-#define REVIVER(ptr)	(is_rider(ptr) || ptr->mlet == S_TROLL)
-
 #define KEEPTRAITS(mon)	(mon->isshk || mon->mtame || \
-			 (mon->data->geno & G_UNIQ) || REVIVER(mon->data) || \
+			 (mon->data->geno & G_UNIQ) || is_reviver(mon->data) || \
 			 (mon->m_id == quest_status.leader_m_id))
 
 /* Creates a monster corpse, a "special" corpse, or nothing if it doesn't