const-qualify ItemPointer comparison functions

Add const qualifiers to ItemPointerEquals() and ItemPointerCompare().
This will allow further changes up the stack.  It also complements
commit aeb767ca0b, as we now have all of itemptr.h appropriately
const-qualified.

Author: Chao Li <li.evan.chao@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CAEoWx2nBaypg16Z5ciHuKw66pk850RFWw9ACS2DqqJ_AkKeRsw@mail.gmail.com

src/backend/storage/page/itemptr.c

@@ -32,7 +32,7 @@ StaticAssertDecl(sizeof(ItemPointerData) == 3 * sizeof(uint16),
  *	Asserts that the disk item pointers are both valid!
  */
 bool
-ItemPointerEquals(ItemPointer pointer1, ItemPointer pointer2)
+ItemPointerEquals(const ItemPointerData *pointer1, const ItemPointerData *pointer2)
 {
 	if (ItemPointerGetBlockNumber(pointer1) ==
 		ItemPointerGetBlockNumber(pointer2) &&
@@ -48,7 +48,7 @@ ItemPointerEquals(ItemPointer pointer1, ItemPointer pointer2)
  *		Generic btree-style comparison for item pointers.
  */
 int32
-ItemPointerCompare(ItemPointer arg1, ItemPointer arg2)
+ItemPointerCompare(const ItemPointerData *arg1, const ItemPointerData *arg2)
 {
 	/*
 	 * Use ItemPointerGet{Offset,Block}NumberNoCheck to avoid asserting

src/include/storage/itemptr.h

@@ -217,8 +217,8 @@ ItemPointerSetMovedPartitions(ItemPointerData *pointer)
  * ----------------
  */
 
-extern bool ItemPointerEquals(ItemPointer pointer1, ItemPointer pointer2);
-extern int32 ItemPointerCompare(ItemPointer arg1, ItemPointer arg2);
+extern bool ItemPointerEquals(const ItemPointerData *pointer1, const ItemPointerData *pointer2);
+extern int32 ItemPointerCompare(const ItemPointerData *arg1, const ItemPointerData *arg2);
 extern void ItemPointerInc(ItemPointer pointer);
 extern void ItemPointerDec(ItemPointer pointer);