Fix style violations in syscache lookups.

Project style is to check the success of SearchSysCacheN and friends
by applying HeapTupleIsValid to the result.  A tiny minority of calls
creatively did it differently.  Bring them into line with the rest.

This is just cosmetic, since HeapTupleIsValid is indeed just a null
check at the moment ... but that may not be true forever, and in any
case it puts a mental burden on readers who may wonder why these
call sites are not like the rest.

Back-patch to v11 just to keep the branches in sync.  (The bulk of these
errors seem to have originated in v11 or v12, though a few are old.)

Per searching to see if anyplace else had made the same error
repaired in 62148c352.

src/backend/commands/operatorcmds.c

@@ -407,7 +407,7 @@ AlterOperator(AlterOperatorStmt *stmt)
 	oprId = LookupOperWithArgs(stmt->opername, false);
 	catalog = table_open(OperatorRelationId, RowExclusiveLock);
 	tup = SearchSysCacheCopy1(OPEROID, ObjectIdGetDatum(oprId));
-	if (tup == NULL)
+	if (!HeapTupleIsValid(tup))
 		elog(ERROR, "cache lookup failed for operator %u", oprId);
 	oprForm = (Form_pg_operator) GETSTRUCT(tup);