[llvm] r233447 - [SCEV] Look at backedge dominating conditions.

[Sanjoy Das]

> The isSingleEdge() call you make in this patch will be linear time, sadly ;(

Yes, looks like isSingleEdge is O(# of successors).  It should be
possible to write a faster but less precise version of isSingleEdge
such that it only looks at blocks with two successors.  I think I can
also exploit LoopInfo::isLoopExiting to make this faster.

However, before I do any of the above, I want to understand what
actually is causing the slowdown. :)

-- Sanjoy