pgsql: Push the responsibility for handling ignore_killed_tuples down

Log Message:
-----------
Push the responsibility for handling ignore_killed_tuples down into
_bt_checkkeys(), instead of checking it in the top-level nbtree.c routines
as formerly. This saves a little bit of loop overhead, but more importantly
it lets us skip performing the index key comparisons for dead tuples.

Modified Files:
--------------
pgsql/src/backend/access/nbtree:
nbtree.c (r1.134 -> r1.135)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtree.c.diff?r1=1.134&r2=1.135)
nbtsearch.c (r1.98 -> r1.99)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtsearch.c.diff?r1=1.98&r2=1.99)
nbtutils.c (r1.66 -> r1.67)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtutils.c.diff?r1=1.66&r2=1.67)
pgsql/src/include/access:
nbtree.h (r1.88 -> r1.89)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/nbtree.h.diff?r1=1.88&r2=1.89)