Eigen pblend

Reference issue

What does this implement/fix?

While addressing some observations by @cantonios , I took this opportunity to apply some lessons learned to clean up the Intel pblend implementations.

• converting an array of bool to a mask is efficiently accomplished by subtracting the bool (in integer form) from zero.
• there is no reason to use floating point comparisons to create bit masks. integer math and comparisons are much faster
• _mm_movemask_epi8 efficiently converts a byte mask into a true bit mask. https://godbolt.org/z/eMsqrz4rM
• defer to pselect instead of spamming macro guards

Additional information