At the risk of bike-shedding...

The fact that "nonexistent" is in regular use doesn't mean that
"existent" is entirely kosher:
http://www.ling.upenn.edu/courses/ling001/mccord.html

In the 'sz' fields, I think "nonexistent" is right: it's zero when the
registers don't exist.

But in rs6000_gdbarch_init, "extant" is a much better word, having the
connotations of "already".

I've committed the attached.