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 <kretz@kde.org>
SSE simd_cast: add missing Vc_INTRINSIC

this is an inline helper function - it showed up with multiple
definitions at link time...

Signed-off-by: Matthias Kretz <kretz@kde.org>
Scalar::Mask: fix generate impl

The conversion from bool to Mask is not implicit.

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