[clang] Fixed vec_pack_to_short_fp32 in Clang altivec.h (PR #129923)

[John Platts via cfe-commits]

johnplatts wrote:

I have discovered that there is a bug in the VSX `vec_pack_to_short_fp32(a, b)` intrinsic with GCC on big-endian POWER9 (but not little-endian POWER9), where `vec_pack_to_short_fp32(a, b)` returns `{b[0], b[1], b[2], b[3], a[0], a[1], a[2], a[3]}` instead of the expected `{a[0], a[1], a[2], a[3], b[0], b[1], b[2], b[3]}`.

PVIPR expects the `vec_pack_to_short_fp32(a, b)` intrinsic to return the same results on both big-endian POWER9 and little-endian POWER9.

https://github.com/llvm/llvm-project/pull/129923