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