x86: correct and simplify NOP disassembly

x86: correct and simplify NOP disassembly

It's not just REX.W which is ignored with opcode 0x90. The same goes for
REX.R and REX.X as well as empty REX. None of these are forms of
"xchg %eax,%eax" (which would mean zero-extending %eax to %rax), so they
also shouldn't be disassembled this way.

While there simplify things: A single hook function suffices, thus
making it unnecessary to keep two expressions in sync. And checking
ins->address_mode for mode_64bit also is unnecessary, as "rex" can be
non-zero only in that case anyway.