pgsql: This patch refactors away some duplicated code in the index AM

Log Message:
-----------
This patch refactors away some duplicated code in the index AM build
methods: they all invoke UpdateStats() since they have computed the
number of heap tuples, so I created a function in catalog/index.c that
each AM now calls.

Modified Files:
--------------
pgsql/src/backend/access/gist:
gist.c (r1.113 -> r1.114)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gist.c.diff?r1=1.113&r2=1.114)
pgsql/src/backend/access/hash:
hash.c (r1.78 -> r1.79)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/hash/hash.c.diff?r1=1.78&r2=1.79)
pgsql/src/backend/access/nbtree:
nbtree.c (r1.129 -> r1.130)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtree.c.diff?r1=1.129&r2=1.130)
pgsql/src/backend/access/rtree:
rtree.c (r1.88 -> r1.89)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/rtree/rtree.c.diff?r1=1.88&r2=1.89)
pgsql/src/backend/catalog:
index.c (r1.254 -> r1.255)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/index.c.diff?r1=1.254&r2=1.255)
pgsql/src/include/catalog:
index.h (r1.62 -> r1.63)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/index.h.diff?r1=1.62&r2=1.63)