Parser: add where clause when parsing an index

[brianbroderick]

CREATE INDEX idx_temp_stuff ON my_temp USING btree( id, temp_id ) WHERE ( blah_id = 1 );

It's not currently parsing the where portion of the statement. Instead, it's throwing that into a 2nd statement with a WHERE prefix expression.

[brianbroderick]

2f1aebf09c9f2f64651b3d998faeaf4167e4186f