nbtree: Cache operator family OID in _bt_setup_array_cmp

Hi Hackers,

While reviewing a separate patch related to nbtree, I noticed
that _bt_setup_array_cmp in nbtpreprocesskeys.c performs multiple redundant
lookups of the operator family OID via rel->rd_opfamily[skey->sk_attno - 1].

The attached patch caches this value in a local opfamily variable. This
matches the pattern used in several other functions within the same file
(such as _bt_skiparray_strat_decrement, _bt_preprocess_array_keys, etc.),
making the code more consistent and readable.

The assignment is placed after the early-return check for (elemtype ==
opcintype) to ensure we only perform the array indexing when the cross-type
comparison logic is actually reached.

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