Big changes. Interval*.h is now more or less finalized. IntervalPartition

is recoded to use IntervalIterators.  IntervalIterators can now maintain
their own memory or let an external entity do it.

Loop depth is a new user of IntervalPartition for calculating the loop
nesting depth of a basic block

TODO: add IntervalPartition capability to split intervals between the looping
portion and the "tail" portion.

llvm-svn: 69