Comment by userbinator

Unfortunately they (AMD) didn't reassign those opcodes for some other purpose - they've just become completely invalid.

Instead, a whole row of useful general-purpose increment and decrement instructions was replaced by 16 REX prefices. A bit odd if you consider that the number of BCD and segment prefix opcodes they made invalid would've been more than enough to be assigned to the new REXes, and still maintain a consistent encoding...