[PATCH] [InstCombine] mark ADD with nuw if no unsigned overflow

[Jingyue Wu]

Hi eliben, meheff, rafael,

As a starting step, we only use one simple heuristic: if the sign bits
of both a and b are zero, we can prove "add a, b" do not unsigned
overflow, and thus convert it to "add nuw a, b".

Updated all affected tests and added two new tests (@zero_sign_bit and
@zero_sign_bit2) in AddOverflow.ll

http://reviews.llvm.org/D4144

Files:
  lib/Transforms/InstCombine/InstCombine.h
  lib/Transforms/InstCombine/InstCombineAddSub.cpp
  test/Transforms/InstCombine/AddOverFlow.ll
  test/Transforms/InstCombine/add-sitofp.ll
  test/Transforms/InstCombine/ffs-1.ll
  test/Transforms/InstCombine/select.ll
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