AVX2: fix horizontal reductions (return type)
AVX2: fix horizontal reductions (return type)

* The return type was incorrectly set to float instead of the template
parameter T. This lead to loss of precision.
* Additionally, use Common::IntrinsicType to simplify the argument type.

Refs: gh-11
Signed-off-by: Matthias Kretz <372ea4eca737a13cd010ee806d4ce7b98ee71550@kde.org>

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