llvmpipe: fix arguments order given to vec_andc

llvmpipe: fix arguments order given to vec_andc

This patch fixes a classic "confuse the enemy" bug.

_mm_andnot_si128 (SSE) and vec_andc (VMX) do the same operation, but the
arguments are opposite.

_mm_andnot_si128 performs "r = (~a) & b" while
vec_andc performs "r = a & (~b)"

To make sure this error won't return in another place, I added a wrapper
function, vec_andnot_si128, in u_pwr8.h, which makes the swap inside.

Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>