nbtree: Remove useless 'strat' local variable.

Remove a local variable that was used to avoid overwriting strat_total
with the = operator strategy when a >= operator strategy key was already
included in the initial positioning/insertion scan keys by _bt_first
(for backwards scans it would have to be a <= key that was included).
_bt_first's strat_total local variable now simply tracks the operator
strategy of the final scan key that was included in the scan's insertion
scan key (barring the case where the !used_all_subkeys row compare path
adjusts strat_total in its own way).

_bt_first already treated >= keys (or <= keys) as = keys for initial
positioning purposes.  There is no good reason to remember that that was
what happened; no later _bt_first step cares about the distinction.
Note, in particular, that the insertion scan key's 'nextkey' and
'backward' fields will be initialized the same way regardless.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Tomas Vondra <tomas@vondra.me>
Discussion: https://postgr.es/m/CAH2-Wz=PKR6rB7qbx+Vnd7eqeB5VTcrW=iJvAsTsKbdG+kW_UA@mail.gmail.com

Author: petergeoghegan

src/backend/access/nbtree/nbtsearch.c

@@ -883,7 +883,6 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
 	BTScanOpaque so = (BTScanOpaque) scan->opaque;
 	BTStack		stack;
 	OffsetNumber offnum;
-	StrategyNumber strat;
 	BTScanInsertData inskey;
 	ScanKey		startKeys[INDEX_MAX_KEYS];
 	ScanKeyData notnullkeys[INDEX_MAX_KEYS];
@@ -1090,18 +1089,11 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
 					break;
 				startKeys[keysz++] = chosen;
 
-				/*
-				 * Adjust strat_total, and quit if we have stored a > or <
-				 * key.
-				 */
-				strat = chosen->sk_strategy;
-				if (strat != BTEqualStrategyNumber)
-				{
-					strat_total = strat;
-					if (strat == BTGreaterStrategyNumber ||
-						strat == BTLessStrategyNumber)
-						break;
-				}
+				/* Quit if we have stored a > or < key */
+				strat_total = chosen->sk_strategy;
+				if (strat_total == BTGreaterStrategyNumber ||
+					strat_total == BTLessStrategyNumber)
+					break;
 
 				/*
 				 * Done if that was the last attribute, or if next key is not