If SETO/SETUO aren't legal, they'll be expanded and we'll end up
with 3 comparisons.
SETONE is equivalent to (SETOGT || SETOLT)
so if one of those operations is supported use that expansion. We
don't need both since we can commute the operands to make the other.
SETUEQ can be implemented with !(SETOGT || SETOLT) or (SETULE && SETUGE).
I've only implemented the first because it didn't look like most of the
affected targets had legal SETULE/SETUGE.
Minor, but maybe we could put into the comments the part of the commit message explaining why only one of OGT/OLT have to be legal for this to be worthwhile? I think that's quite useful background for anyone coming across this in the future.