[PATCH] Fix bug 22641

[Sanjoy Das]

Hi atrick, hfinkel, spatel,

Fix bug 22641 by partially reinstating the original change for D7331.

Point to note: 22641 is _not_ caused by SCEV "forgetting" control flow.  `PreAR` is marked as `nuw` by SCEV itself when it sees that `Start + BECount * Step` won't overflow for `PreAR` (since `BECount` is `0`).

http://reviews.llvm.org/D7808

Files:
  lib/Analysis/ScalarEvolution.cpp
  test/Analysis/ScalarEvolution/infer-prestart-no-wrap.ll
  test/Analysis/ScalarEvolution/pr22641.ll
  test/Analysis/ScalarEvolution/scev-prestart-nowrap.ll

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