A region identifies a set of vector instructions generated by
vectorization passes. The vectorizer can then run a series of
RegionPasses on the region, evaluate the cost, and commit/reject the
transforms on a region-by-region basis, instead of an entire basic
block.
This is heavily based ov @vporpo's prototype. In particular, the doc
comment for the Region class is all his. The rest of this commit is
mostly boilerplate around a SetVector: getters, iterators, and some
debug helpers.
