[SCEV] Extract out a isSCEVExprNeverPoison helper; NFCI

Summary:
Also adds a small comment blurb on control flow + no-wrap flags, since
that question came up a few days back on llvm-dev.

Reviewers: bjarke.roune, broune

Subscribers: sanjoy, mcrosier, llvm-commits, mzolotukhin

Differential Revision: http://reviews.llvm.org/D19209