x86: move reg_operands adjustment

Ideally we'd do away with this somewhat questionable adjustment (leaving
i.types[] untouched). That's non-trivial though as it looks, so only
- move the logic into process_operands(), putting it closer to related
  logic and eliminating a conditional for operand-less insns,
- make it consistent (i.e. also affect %xmm0), eliminating an ugly
  special case later in the function.