[PATCH] IndVarSimplify: Don't let LFTR compare against a poison value

[David Majnemer]

Hi chandlerc, nicholas, nlewycky, atrick,

LinearFunctionTestReplace tries to use the *next* indvar to compare
against when possible.  However, it may be the case that the calculation
for the next indvar has NUW/NSW flags and that it may only be safely
used inside the loop.  Using it in a comparison to calculate the exit
condition could result in observing poison.

This fixes PR20680.

http://reviews.llvm.org/D5174

Files:
  lib/Transforms/Scalar/IndVarSimplify.cpp
  test/Transforms/IndVarSimplify/2011-10-27-lftrnull.ll
  test/Transforms/IndVarSimplify/lftr-address-space-pointers.ll
  test/Transforms/IndVarSimplify/lftr-extend-const.ll
  test/Transforms/IndVarSimplify/lftr-reuse.ll
  test/Transforms/IndVarSimplify/pr20680.ll
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