casts tests: fix test for MIC

with MIC, the in-register representation can differ from the in-memory
representation, thus the conversion to T here created different values
in the reference vector, even though they were copied directly from the
result vector.
Now it sets both entries to zero instead.

Signed-off-by: Matthias Kretz <kretz@kde.org>
MIC: simd_cast implementation for MIC-MIC

This is a complete, and tested, implementation of simd_cast from MIC
Vector<T> to MIC Vector<T>, including 2->1 and 1->2 casts. The casts
involving shorts were a little tricky as they required shifting or
masking.

Signed-off-by: Matthias Kretz <kretz@kde.org>
ICC: it wants std::size_t instead of size_t

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