Be wary of abnormal exits from loop when exploiting UB

We can safely rely on a NoWrap add recurrence causing UB down the road
only if we know the loop does not have a exit expressed in a way that is
opaque to ScalarEvolution (e.g. by a function call that conditionally
calls exit(0)).

I believe with this change PR28012 is fixed.

Note: I had to change some llvm-lit tests in LoopReroll, since it looks
like they were depending on this incorrect behavior.

llvm-svn: 272237