SCEVExpander incorrectly marks generated subtractions as nuw/nsw

It is not sound to mark the increment operation as `nuw` or `nsw`
based on a proof off of the add recurrence if the increment operation
we emit happens to be a `sub` instruction.

I could not come up with a test case for this -- the cases where
SCEVExpander decides to emit a `sub` instruction is quite small, and I
cannot think of a way I'd be able to get SCEV to prove that the
increment does not overflow in those cases.

Differential Revision: http://reviews.llvm.org/D7899

llvm-svn: 230673