In _bt_check_unique() loop, don't bother applying _bt_isequal() to

In _bt_check_unique() loop, don't bother applying _bt_isequal() to
killed items; just skip to the next item immediately.  Only check for
key equality when we reach a non-killed item or the end of the index
page.  This saves key comparisons when there are lots of killed items,
as for example in a heavily-updated table that's not been vacuumed lately.
Seems to be a win for pgbench anyway.