x86/Intel: correct AVX512 S/G disassembly

Commit 6ff00b5e ("x86/Intel: correct permitted operand sizes for
AVX512 scatter/gather") brought the assembler side of AVX512 S/G insn
handling in line with AVX2's, but the disassembler side was forgotten.
This has the benefit of
- allowing to fold a number of table entries,
- rendering a few #define-s and enumerators unused.