splitting add instructions

[DJ Delorie]

> What's the point of the second define insn? The first insn seems to
> take us from expansion to asm generation so I can't see where the
> second one will come into play except in an expansion after reload
> but that doesn't happen, right?

IIRC it has to do with optimizing away compare instructions.  Which
pattern you need depends on whether the flags will get used by a
conditional branch or not.