I am not sure how tablegen can tell we are not going to use it. That would be a separate patch anyway, since we need a way to point to a pset even if we have fewer of them. My guess the only reason not to have this enum was some non conventional characters in names for some targets.
I was looking for one and didn't find. Looks like in other cases tablegen just avoids such names. Here I saw "+" sign. Probably it will take yet another change to remove both invalid symbols and this function.