[PATCH] Bugfix: SCEV incorrectly marks certain add recurrences as nsw

[Sanjoy Das]

In practice it seems the two places that actually do something with `<nw>` work only with a constant non-zero step size, so it is probably okay to remove the check.  But I think the comments in the header file should be changed to state that `<nw>` means that `step` is either `0` or `abs(step) * max-iteration(loop) <= unsigned-max(bitwidth)`.


http://reviews.llvm.org/D7495

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