x86: emit {evex} prefix when disassembling ambiguous AVX512VL insns

x86: emit {evex} prefix when disassembling ambiguous AVX512VL insns

When no AVX512-specific functionality is in use, the disassembly of
AVX512VL insns is indistinguishable from their AVX counterparts (if such
exist). Emit the {evex} pseudo-prefix in such cases.

Where applicable drop stray uses of PREFIX_OPCODE from table entries.