casts test: replace mask_cast_0_5 with recursive impl
casts test: replace mask_cast_0_5 with recursive impl

new cast_mask_split test function for testing simd_cast<To, N> calls
recursively.

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

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