x86: consistently use scalar_mode for AVX512-FP16 scalar insns

For some reason the original AVFX512F insns were not taken as a basis
here, causing unnecessary divergence. While not an active issue, it is
still relevant to note that OP_XMM() has special treatment of e.g.
scalar_mode (marking broadcast as invalid). Such would better be
consistent for all sufficiently similar insns.