Tweak indexscan machinery to avoid taking an AccessShareLock on an index

Tweak indexscan machinery to avoid taking an AccessShareLock on an index
if we already have a stronger lock due to the index's table being the
update target table of the query.  Same optimization I applied earlier
at the table level.  There doesn't seem to be much interest in the more
radical idea of not locking indexes at all, so do what we can ...