[PATCH] [SCEV] make SCEV smarter about proving no-wrap.

[Sanjoy Das]

Addresses review.

When writing down a proof for `proveNoWrapByVaryingStart` (thanks for
prodding me to do that!) I realized that the first version had a bug

- we need to prove (S-X)+X does not overflow for the inference to be

valid.  This current version addresses that.

The current version also swaps out a full SCEVAddRecExpr construction
and just looks at the UniqueSCEVs set instead, and gives up if there
isn't a pre-existing add recurrence of the form we need.  This should
address the compile time issue.


http://reviews.llvm.org/D7980

Files:
  include/llvm/Analysis/ScalarEvolution.h
  lib/Analysis/ScalarEvolution.cpp
  test/Analysis/ScalarEvolution/nowrap-preinc-limits.ll

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