Revive fallback impl for older compilers
Revive fallback impl for older compilers

... that complain about _mm256_shuffle_epi8 and _mm256_permute4x64_epi64
usage when Vc_IMPL_AVX2 is not defined

Signed-off-by: Matthias Kretz <372ea4eca737a13cd010ee806d4ce7b98ee71550@kde.org>

VcDevel/Vc,VcDevel/Vc,VcDevel/Vc,VcDevel/Vc