pgsql: Fix broken cast on MSVC

Fix broken cast on MSVC

Per buildfarm animal drongo, casting a vector type to the same type
causes a compile error. We still need the cast on ARM64, so invent a
wrapper function that does the casting only where necessary.

Discussion:
https://www.postgresql.org/message-id/CAFBsxsEouaTwbmpqV%2BEW2%3DwFbhw2vHRe26NQTRcd0%3DNaOFDy7A%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c6a43c25a8ba1a825588f7af25a7173f2e66ce8d

Modified Files
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src/include/port/pg_lfind.h |  2 +-
src/include/port/simd.h     | 22 ++++++++++++++++++++++
2 files changed, 23 insertions(+), 1 deletion(-)