gallivm: use pmulhrsw to make aos sampling more accurate.

gallivm: use pmulhrsw to make aos sampling more accurate.

This uses pmulhrsw avx2 and ssse3 variants. It fixes the
precision of texture filtering calculations.

However it does leave these paths inaccurate on platforms
that don't support it.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Acked-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13075>