[PECOFF] Treat the common symbol as mergeable symbol.

This is a temporary measure because the semantics of the common symbol is
actually more compilcated than the simple mergeable symbol.
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.