Remove IndexIsValid(), IndexIsReady(), IndexIsLive() in favor of
accessing the index structure directly.  These macros haven't been
used consistently, and the original reason of maintaining source
compatibility with PostgreSQL 9.2 is gone.

Discussion: https://www.postgresql.org/message-id/flat/d419147c-09d4-6196-5d9d-0234b230880a%402ndquadrant.com