[SROA] Fix a *glaring* bug in r202091: you have to actually *write*
the break statement, not just think it to yourself....

No idea how this worked at all, much less survived most bots, my
bootstrap, and some bot bootstraps!

The Polly one didn't survive, and this was filed as PR18959. I don't
have a reduced test case and honestly I'm not seeing the need. What we
probably need here are better asserts / debug-build behavior in
SmallPtrSet so that this madness doesn't make it so far.