Avoid unnecessary page-level SSI lock check in heap_insert().

As observed by Heikki, we need not conflict on heap page locks during an
insert; heap page locks are only aggregated tuple locks, they don't imply
locking "gaps" as index page locks do.  So we can avoid some unnecessary
conflicts, and also do the SSI check while not holding exclusive lock on
the target buffer.

Kevin Grittner, reviewed by Jeff Davis.  Back-patch to 9.1.