Speedup WindowAgg code by moving uncommon code out-of-line

Speedup WindowAgg code by moving uncommon code out-of-line

The code to calculate the frame offsets is only performed once per scan.
Moving this code out of line gives a small (around 4-5%) speedup when testing
with some CPUs.  Other tested CPUs are indifferent to the change.

Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reviewed-by: Tatsuo Ishii <ishii@postgresql.org>
Discussion: https://postgr.es/m/CAApHDvqPgFtwme2Zyf75BpMLwYr2mnUstDyPiP%3DEpudYuQTPPQ%40mail.gmail.com