[FFmpeg-devel] [PATCH V3] swscale/ppc/yuv2rgb_altivec: Fixes compiler bug - replace vec_lvsl/vec_perm with vec_xl

[ckerchne]

A bug exist with the gcc compilers for Power in versions 6.x and 7.x 
(verified with 6.3 and 7.4). It was fixed in version 8.x (verified with 
8.3). I was using a Power 9 ppc64le machine for building and testing.

It appears the compiler is generating the wrong code for little endian 
machines for the vec_lvsl/vec_perm instruction combos in some cases. If 
these instructions are replaced with vec_xl, the problem goes away for 
all versions of the compilers (plus it is a bit faster).

Fixes #7124 and can be reproduced within the docker by following the 
instructions within the ticket.

https://trac.ffmpeg.org/ticket/7124
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