0x86 and 0x87 (XCHG rm,r with byte- and word-sized operands) encode nops in the same way as 0x90, but were never redefined as prefixes.

It's interesting that a lot of ALU operations occupy four opcodes (memory source/memory destination x byte/word) but 0x84/0x85 and 0x86/0x87 only need two because not only are they commutative, but also 0x84/0x85 do not write to any operands and 0x86/0x87 write to both. So there is no difference between memory as the source or destination operand.