pgsql: Frob numeric.c loop so that clang will auto-vectorize it too.

Frob numeric.c loop so that clang will auto-vectorize it too.

Experimentation shows that clang will auto-vectorize the critical
multiplication loop if the termination condition is written "i2 < limit"
rather than "i2 <= limit".  This seems unbelievably stupid, but I've
reproduced it on both clang 9.0.1 (RHEL8) and 11.0.3 (macOS Catalina).
gcc doesn't care, so tweak the code to do it that way.

Discussion: https://postgr.es/m/CAJ3gD9evtA_vBo+WMYMyT-u=keHX7-r8p2w7OSRfXf42LTwCZQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9c79e646c6f0f8df06d966c536d0c6aa33bf1b06

Modified Files
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src/backend/utils/adt/numeric.c | 14 ++++++++------
1 file changed, 8 insertions(+), 6 deletions(-)