Hi,

With this patch, it is OK for find_implicit_sets to split an edge if
that makes it possible to record an implicit set on that edge. This is
possible and cheap now because CPROP runs in cfglayout mode.

Bootstrapped and tested on x86_64-unknown-linux-gnu. Shaves off ~1kb
of cc1, which isn't very much but still kind of nice.
OK?

Ciao!
Steven