Fix an oversight in isPotentiallyReachable where we wouldn't do any CFG-walking

to find loops if the From and To instructions were in the same block.

Refactor the code a little now that we need to fill to start the CFG-walking
algorithm with more than one starting basic block sometimes.

Special thanks to Andrew Trick for catching an error in my understanding of
natural loops in code review.

llvm-svn: 188236