pgsql: 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(dot)bapat(dot)oss(at)gmail(dot)com>
Reviewed-by: Tatsuo Ishii <ishii(at)postgresql(dot)org>
Discussion: https://postgr.es/m/CAApHDvqPgFtwme2Zyf75BpMLwYr2mnUstDyPiP%3DEpudYuQTPPQ%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/19b861f880166fbdb67d268955e590881376f876

Modified Files
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src/backend/executor/nodeWindowAgg.c | 140 +++++++++++++++++++----------------
1 file changed, 78 insertions(+), 62 deletions(-)