pgsql: Abstract some more architecture-specific details away from SIMD

Abstract some more architecture-specific details away from SIMD functionality

Add a typedef to represent vectors containing four 32-bit integers,
and add functions operating on them. Also separate out saturating
subtraction into its own function. The motivation for this is to
prepare for a future commit to add ARM NEON support.

Nathan Bossart

Reviewed by John Naylor and Tom Lane
Discussion:
https://www.postgresql.org/message-id/flat/CAFBsxsEyR9JkfbPcDXBRYEfdfC__OkwVGdwEAgY4Rv0cvw35EA%40mail.gmail.com#aba7a64b11503494ffd8dd27067626a9

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f8f19f70868c6351b80f02a47fa65d56b728a5a2

Modified Files
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src/include/port/pg_lfind.h | 65 +++++++++++++++++------------
src/include/port/simd.h     | 99 +++++++++++++++++++++++++++++++++++++--------
2 files changed, 121 insertions(+), 43 deletions(-)