Don't try to fix eliminated nbtree array scan keys.

Don't try to fix eliminated nbtree array scan keys.

Preprocessing for nbtree index scans allowed array "input" scan keys
already marked eliminated during array-specific preprocessing to be
"fixed up" during preprocessing proper.  This allowed eliminated scan
keys on DESC index columns to spurious have their strategy commuted,
causing assertion failures.

To fix, teach _bt_fix_scankey_strategy to ignore these scan keys.  This
brings it in line with its only caller, _bt_preprocess_keys.

Oversight in commit 5bf748b8, which enhanced nbtree ScalarArrayOp
execution.

Reported-By: Donghang Lin <donghanglin@gmail.com>
Discussion: https://postgr.es/m/CAA=D8a2sHK6CAzZ=0CeafC-Y-MFXbYxnRSHvZTi=+JHu6kAa8Q@mail.gmail.com