Allow to specify a index name as ANALYZE parameter

Hi,

When we specify column names for ANALYZE, only the statistics for those columns
are collected. Similarly, is it useful if we have a option to specify an index
for ANALYZE to collect only the statistics for expression in the specified index?

A usecase I suppose is that when a new expression index is created and that
we need only the statistics for the new index. Although ANALYZE of all the indexes
is usually fast because ANALYZE uses a random sampling of the table rows, ANALYZE
on the specific index may be still useful if there are other index whose "statistics
target" is large and/or whose expression takes time to compute, for example.

Attached is the WIP patch to allow to specify a index name as ANALYZE parameter.
Any documatation is not yet included. I would appreciate any feedback!

Regards,

--
Yugo Nagata <nagata(at)sraoss(dot)co(dot)jp>