Fix incorrect const qualification for tbm_add_tuples() and itemptr_to_uint64()

Hi Hackers,

I noticed a wrong const qualification:
```
void
tbm_add_tuples(TIDBitmap *tbm, const ItemPointer tids, int ntids,
  bool recheck)
```

This "const" only protects "tids" itself from updating, which is
meaningless. I believe the real intention should be protecting the content
"tids" pointing to from updating.

This behavior can be easily proved by the compiler. If we add a line of
fake code in the function:
```
tids[0].ip_posid = 0;
```

With current "const ItemPointer tids", the compiler won't report any
problem. If we change to "const ItemPointerData *tids", the compiler will
raise an error due to the assignment to read-only variable.

I searched over the source tree, and found only one more occurrence in
itemptr_to_uint64(), so I fixed it as well.

Also, as I am touching tbm_add_tuples(), I did a tiny change that moved the
loop variable "i" into "for". Peter Eisentraut just did the same change in
formatting.c [1].

[1]
https://git.postgresql.org/cgit/postgresql.git/commit/?id=03fbb0814c5015ab79e670ab97bb6a3349269e4b

Best regards,
Chao Li (Evan)
---------------------
HighGo Software Co., Ltd.
https://www.highgo.com/