simd_cast: add debug trace output
simd_cast: add debug trace output

since simd_cast implementations depend on relatively deep nesting of
simd_cast calls it is very helpful to trace the calls to understand
where something goes wrong. Thanks to variadic templates this is
non-trivial to do, therefore I developed a helper function that does the
trick and can be enabled via the VC_DEBUG_SIMD_CAST macro.

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

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