Un-special-case "xchg rax,rax"; allow it to be encoded as 48 90 for
orthogonality's sake. It's a no-op, to be sure, but so are many other
instructions.
"xchg eax,eax" is still special-cased in 64-bit mode since it is not a
no-op; unadorned opcode 90 is now simply "nop" and nothing else.
Make the disassembler detect unused REX.W and display them as an "o64"
prefix.
