Cleanup some aggregate code in the executor

Cleanup some aggregate code in the executor

Here we alter the code that calls build_pertrans_for_aggref() so that the
function no longer needs to special-case whether it's dealing with an
aggtransfn or an aggcombinefn.  This allows us to reuse the
build_aggregate_transfn_expr() function and just get rid of the
build_aggregate_combinefn_expr() completely.

All of the special case code that was in build_pertrans_for_aggref() has
been moved up to the calling functions.

This saves about a dozen lines of code in nodeAgg.c and a few dozen more
in parse_agg.c

Also, rename a few variables in nodeAgg.c to try to make it more clear
that we're working with either a aggtransfn or an aggcombinefn.  Some of
the old names would have you believe that we were always working with an
aggtransfn.

Discussion: https://postgr.es/m/CAApHDvptMQ9FmF0D67zC_w88yVnoNVR2+kkOQGUrCmdxWxLULQ@mail.gmail.com