Fix masked gather of SSE::(u)short_v
Fix masked gather of SSE::(u)short_v

... and simplify the implementation

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

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