Rework SIMD zeroing

[TDecking]

This PR adds a dedicated ZERO constant for the internal SIMD types and uses it intead of current idioms such as splat(0) or setzero_pd().as_<type>(). The result is improved code generation in non-optimized builds as well as a small line count reduction.

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[Amanieu]

CI should be fixed now, can you rebase?