pgsql: Add a back-link from IndexOptInfo structs to their parent

Log Message:
-----------
Add a back-link from IndexOptInfo structs to their parent RelOptInfo
structs. There are many places in the planner where we were passing
both a rel and an index to subroutines, and now need only pass the
index struct. Notationally simpler, and perhaps a tad faster.

Modified Files:
--------------
pgsql/src/backend/optimizer/path:
costsize.c (r1.138 -> r1.139)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/costsize.c.diff?r1=1.138&r2=1.139)
indxpath.c (r1.170 -> r1.171)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/indxpath.c.diff?r1=1.170&r2=1.171)
orindxpath.c (r1.66 -> r1.67)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/orindxpath.c.diff?r1=1.66&r2=1.67)
pathkeys.c (r1.64 -> r1.65)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/pathkeys.c.diff?r1=1.64&r2=1.65)
pgsql/src/backend/optimizer/plan:
createplan.c (r1.176 -> r1.177)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/createplan.c.diff?r1=1.176&r2=1.177)
pgsql/src/backend/optimizer/util:
pathnode.c (r1.113 -> r1.114)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/pathnode.c.diff?r1=1.113&r2=1.114)
plancat.c (r1.101 -> r1.102)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/plancat.c.diff?r1=1.101&r2=1.102)
pgsql/src/include/nodes:
relation.h (r1.103 -> r1.104)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/relation.h.diff?r1=1.103&r2=1.104)
pgsql/src/include/optimizer:
cost.h (r1.62 -> r1.63)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/optimizer/cost.h.diff?r1=1.62&r2=1.63)
pathnode.h (r1.56 -> r1.57)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/optimizer/pathnode.h.diff?r1=1.56&r2=1.57)
paths.h (r1.79 -> r1.80)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/optimizer/paths.h.diff?r1=1.79&r2=1.80)