(... 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, 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.
: i.e. cases where block A branches to block B and B is A's only
successor and A is B's only predecessor.