x86: allow use of broadcast with X/Y/Z-suffixed AVX512-FP16 insns

x86: allow use of broadcast with X/Y/Z-suffixed AVX512-FP16 insns

While the x/y/z suffix isn't necessary to use in this case, it is still
odd that these forms don't support broadcast (unlike their AVX512F /
AVX512DQ counterparts). The lack thereof can e.g. make macro-ized
programming more difficult.