traits: make IsLoadStoreFlag work for Prefetch<>
traits: make IsLoadStoreFlag work for Prefetch<>

even if Prefetch<...> derives from LoadStoreFlags this specialization
didn't match. Added a second specialization to catch it.

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

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