Add mf8 forms of vbsl, vluti2 and vluti4 (#372)

vbsl has floating-point forms, since the intrinsic can be useful for
blending two different sources. This patch adds mf8 forms too, with the
same justification.

Also, the FEAT_LUT support was done in parallel with the FP8 support.
The 8-bit forms make sense for mf8 too.