simd_cast: fix implementation selection
simd_cast: fix implementation selection

The enable_if logic was slightly off and lead to important input values
getting dropped before the actual converting calls were made. Now the
logic should be correct and never drop necessary data.

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

VcDevel/Vc,chr-engwer/Vc,chr-engwer/Vc,chr-engwer/Vc,VcDevel/Vc,VcDevel/Vc,VcDevel/Vc