simdize: make the broadcast ctor more greedy
simdize: make the broadcast ctor more greedy

the perfect forward ctor was too greedy because of the universal
reference arguments. Thus only const lvalue ref arguments worked for the
broadcast constructor. Now the logic is corrected thus that all objects
that are convertible to a ScalarBase type go to the broadcast
constructor.

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

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