simdize: automatically convert mask for each member
simdize: automatically convert mask for each member

simdize<tuple<int, double>> results in tuple<SSE::int_v, AVX::double_v>
for the AVX ABI. Both vector types have a size() of 4. But, their mask
types are not implicitly convertible. Conversion requires a simd_cast.
Therefore, on conditional assignment to a simdize Adapter, enforce that
the mask size matches via enable_if and then use simd_cast to create a
usable mask object for the conditional assignment of the member.

Refs: gh-12

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

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