vc1_inv_trans_8x4_altivec() is supposed to process a block
of 8x4 words, yet it read and processed eight lines. This led
to ASAN failures (see [1]) that this commit intends to fix.
It should also lead to performance improvements, but I don't have
real hardware to bench it.
[1]: https://fate.ffmpeg.org/report.cgi?time=20251207214004&slot=ppc64-linux-gcc-14.3-asan
Reviewed-by: Sean McGovern <gseanmcg@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
