The GPDB "prelim" functions did the same things as the "combine"
functions introduced in PostgreSQL 9.6 This commit includes just the
catalog changes, to essentially search & replace "prelim" with
"combine". I did not pick the planner and executor changes that were
made as part of this in the upstream, yet.

Also replace the GPDB implementation of float8_amalg() and
float8_regr_amalg(), with the upstream float8_combine() and
float8_regr_combine(). They do the same thing, but let's use upstream
functions where possible.

Upstream commits:
commit a7de3dc5
Author: Robert Haas <rhaas@postgresql.org>
Date:   Wed Jan 20 13:46:50 2016 -0500

    Support multi-stage aggregation.

    Aggregate nodes now have two new modes: a "partial" mode where they
    output the unfinalized transition state, and a "finalize" mode where
    they accept unfinalized transition states rather than individual
    values as input.

    These new modes are not used anywhere yet, but they will be necessary
    for parallel aggregation.  The infrastructure also figures to be
    useful for cases where we want to aggregate local data and remote
    data via the FDW interface, and want to bring back partial aggregates
    from the remote side that can then be combined with locally generated
    partial aggregates to produce the final value.  It may also be useful
    even when neither FDWs nor parallelism are in play, as explained in
    the comments in nodeAgg.c.

    David Rowley and Simon Riggs, reviewed by KaiGai Kohei, Heikki
    Linnakangas, Haribabu Kommi, and me.

commit af025eed
Author: Robert Haas <rhaas@postgresql.org>
Date:   Fri Apr 8 13:44:50 2016 -0400

    Add combine functions for various floating-point aggregates.

    This allows parallel aggregation to use them.  It may seem surprising
    that we use float8_combine for both float4_accum and float8_accum
    transition functions, but that's because those functions differ only
    in the type of the non-transition-state argument.

    Haribabu Kommi, reviewed by David Rowley and Tomas Vondra