Common: remove unused types/traits

Signed-off-by: Matthias Kretz <kretz@kde.org>
Scalar/SSE/AVX: refactor common code

All Mask classes need the same decision logic for implicit vs. explicit
casts. This is now in common/types.h. Also, Scalar::Mask used a slightly
different (older) logic and now is up to date.

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