Have isKnownNotFullPoison be smarter around control flow

Summary:
(... while still not using a PostDomTree)

The way we use isKnownNotFullPoison from SCEV today, the new CFG walking
logic will not trigger for any realistic cases -- it will kick in only
for situations where we could have merged the contiguous basic blocks
anyway[0], since the poison generating instruction dominates all of its
non-PHI uses (which are the only uses we consider right now).

However, having this change in place will allow a later bugfix to break
fewer llvm-lit tests.

[0]: i.e. cases where block A branches to block B and B is A's only
successor and A is B's only predecessor.

Reviewers: broune, bjarke.roune

Subscribers: mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D19212

llvm-svn: 267175