x86: correct decoding of nop/reserved space (0f18 ... 0x1f)

x86: correct decoding of nop/reserved space (0f18 ... 0x1f)

All encodings not used in this range are (reserved) NOPs. Hence their
decoding should be fully consistent. For this to work the PREFIX_IGNORED
logic needs slightly extending, such that the attribute will also
- have an effect when used inside prefix_table[], yet without always
  falling back to using slot 0,
- cause prefixes marked as ignored while decoding through prefix_table[]
  to no longer be considered decoded, when encountered in a subsequent
  decoding step.

In adjacent code also drop meaningless PREFIX_OPCODE.