The 2 source operands commutable instructions are encoded in the
VEX.VVVV field and the r/m field of the MODRM byte plus the VEX.B
The VEX.B field is missing from the 2-byte VEX encoding. If the
VEX.VVVV source is 0-7 and the other register is 8-15 we can
swap them to avoid needing the VEX.B field. This works as long as
the VEX.W, VEX.mmmmm, and VEX.X fields are also not needed.