Merging r278999:

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r278999 | hans | 2016-08-17 15:50:18 -0700 (Wed, 17 Aug 2016) | 3 lines

SCEV: Don't assert about non-SCEV-able value in isSCEVExprNeverPoison() (PR28932)

Differential Revision: https://reviews.llvm.org/D23594
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llvm-svn: 279093