Bug: gather/scatter accepts same registers in destination/src and index

[Michael Zolotukhin]

Hi,

> Does AVX2 gather have the similar issue?
For AVX2 gathers we've already had required checks, this patch adds similar
checks for AVX512F      rs

> Shouldn't scatters at least be checked for having an index?
I think it would be checked anyway when we check operands, i.e. when looking for
a matching pattern from i386-opc.tbl (as for all usual instructions).  Please
correct me if I'm wrong here.

Thanks,
Michael