How often have you really needed to do "a = (a & 0xffff00ff) | ((a & 0xff) << 8)"? I don't think I've ever needed to do it, and I wouldn't be surprised if compilers don't even generate "mov ah,al" for that, due to the fact that AH/BH/CH/DH only exist for 4 registers.

Anyway, since you asked: In AArch64 that would be written "bfi w0,w0,#8,#8". "bfi" is an alias of "bfm", an instruction far more flexible and useful than any of the baroque x86 register names. BFM can move an arbitrary number of bits from any position in any register to any other register, and it has optional zero-extension and sign-extension modes.