Avoid unnecessary GinFormTuple() calls for incompressible posting lists.

Avoid unnecessary GinFormTuple() calls for incompressible posting lists.

Previously, we attempted to form a posting list tuple even when
ginCompressPostingList() failed to compress the posting list due to
its size. While there was no functional failure, it always wasted one
GinFormTuple() call when item pointers didn't fit in a posting list
tuple.

This commit ensures that a GIN index tuple is formed only when all
item pointers in the posting list are successfully compressed.

Author: Arseniy Mukhin <[email protected]>
Reviewed-by: Masahiko Sawada <[email protected]>
Discussion: https://postgr.es/m/CAE7r3M+C=jcpTD93f_RBHrQp3C+=TAXFs+k4tTuZuuxboK8AvA@mail.gmail.com