Follow-up from "Make multirhstest compile and run"

The following discussion from !596 (merged) should be addressed:

• @christi started a discussion:

  I'd have to check, but I believe this is really a breaking change in VC, it seems that this matches the following point from the changelog:

  Added Vc::simd<T, Abi> alias that resolves to the corresponding Vc::Vector or Vc::SimdArray type. Most importantly Vc::simd<T, Vc::simd_abi::fixed_size<N>> (alias: Vc::fixed_size_simd<T, N>) will give you (almost) Vc::SimdArray<T, N>. Note that this simd type does not implement the exact same interface as defined in the Parallelism TS v2. Most SimdArray operations return fixed_size_simd now, thus potentially breaking existing code (e.g. by breaking template argument deduction).