MSVC: work around isnan miscompilation
MSVC: work around isnan miscompilation

isnan(SimdArray<double, 8>) miscompiles with only SSE. Using a different
style of initializing the resulting SimdMaskArray works around the
miscompilation.

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

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