Avoid extra lookups with nbtree array inequalities.

Avoid extra lookups with nbtree array inequalities.

nbtree index scans with SAOP inequalities (but no SAOP equalities)
performed extra ORDER proc lookups for any remaining equality strategy
scan keys.  This could waste cycles, and caused assertion failures.
Keeping around a separate ORDER proc is only necessary for a scan's
non-array/non-SAOP equality scan keys when the scan has at least one
other SAOP equality strategy key (a SAOP inequality shouldn't count).

To fix, replace _bt_preprocess_array_keys_final's assertion with a test
that makes the function return early when the scan has no SAOP equality
scan keys.

Oversight in commit 1b134ca5, which enhanced nbtree ScalarArrayOp
execution.

Reported-By: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/0539d3d3-a402-0a49-ed5e-26429dffc4bd@gmail.com