[PATCH] Fix PR22222

[Sanjoy Das]

Hi majnemer, hfinkel, atrick,

The bug was introduced in r225282.  r225282 assumed that `sub X, Y` is the same as `add X, -Y`.  This is not correct if we are going to upgrade the sub to `sub nuw`.  This change fixes the issue by making the optimization ignore `sub` instructions.

http://reviews.llvm.org/D6979

Files:
  lib/Transforms/Utils/SimplifyIndVar.cpp
  test/Transforms/IndVarSimplify/pr22222.ll
  test/Transforms/IndVarSimplify/strengthen-overflow.ll

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