Avoid nbtree index scan SAOP scanBehind confusion.

Consistently reset so->scanBehind at the beginning of nbtree array
advancement, even during sktrig_required=false calls (calls where array
advancement is triggered by an unsatisfied non-required array scan key).
Otherwise, it's possible for queries to fail to return all relevant
tuples to the scan given a low-order required scan key that was
previously deemed "satisfied" by a truncated high key attribute value.
This only happened at the point where a later non-required array scan
key needed to be "advanced" once on the next leaf page (that is, once
the right sibling of the truncated high key page was reached).

The underlying issue was that later code within _bt_advance_array_keys
assumed that the so->scanBehind flag must have been set using the
current page's high key (not the previous page's high key).  Any later
successful recheck call to _bt_check_compare would therefore spuriously
be prevented from making _bt_advance_array_keys return true, based on
the faulty belief that the truncated attribute must be from the scan's
current tuple (i.e. the non-pivot tuple at the start of the next page).
_bt_advance_array_keys would return false for the tuple, ultimately
resulting in _bt_checkkeys failing to return a matching tuple.

Oversight in commit 5bf748b, which enhanced nbtree ScalarArrayOp
execution.

Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-WzkJKncfqyAUTeuB5GgRhT1vhsWO2q11dbZNqKmvjopP_g@mail.gmail.com
Backpatch: 17-, where commit 5bf748b first appears.

Author: petergeoghegan

src/backend/access/nbtree/nbtutils.c

@@ -1813,6 +1813,12 @@ _bt_advance_array_keys(IndexScanDesc scan, BTReadPageState *pstate,
 				all_required_satisfied = true,
 				all_satisfied = true;
 
+	/*
+	 * Unset so->scanBehind (and so->oppositeDirCheck) in case they're still
+	 * set from back when we dealt with the previous page's high key/finaltup
+	 */
+	so->scanBehind = so->oppositeDirCheck = false;
+
 	if (sktrig_required)
 	{
 		/*
@@ -1821,8 +1827,6 @@ _bt_advance_array_keys(IndexScanDesc scan, BTReadPageState *pstate,
 		Assert(!_bt_tuple_before_array_skeys(scan, dir, tuple, tupdesc,
 											 tupnatts, false, 0, NULL));
 
-		so->scanBehind = so->oppositeDirCheck = false;	/* reset */
-
 		/*
 		 * Required scan key wasn't satisfied, so required arrays will have to
 		 * advance.  Invalidate page-level state that tracks whether the