swift234

Add in a post order analysis that lazily recomputes post orders for functions when they are invalidated.

This ensures that if we have a bunch of passes in a row which modify the CFG, we
do not continually rebuild the post order, while at the same time preserving the
property of multiple passes which do not touch the CFG sharing the same post
order, reverse post order rather than recomputing them.

rdar://17654239

Swift SVN r19913