Postgres optimizes the storage of the ItemPointer in the Gin Posting
List. It only stores 11 bits for the offset number because heap tables
only have enough tuples per block to fit in 11 bits. However,
Greenplum append-only tables store 16 bits worth of offset numbers.

Initially we thought we'd need to modify decode_varbyte() but it turns
out that it is OK.  It handles 48 bits already.
Co-authored-by: NAlexandra Wang <lewang@pivotal.com>
Co-authored-by: NAshwin Agrawal <aagrawal@pivotal.io>