Fix AVX512 builds with MSVC.

Antonio Sánchezrequested to merge
cantonios/eigen:msvc into master

Fix AVX512 builds with MSVC.

MSVC doesn't allow reinterpret_cast between types __m512 and __m512d and the like. Requires the use of the explicit cast intrinsic.

Also actually add the AVX512 test option for MSVC for our CI... previously was always testing only SSE.

Fixes #2466 (closed).

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