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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@69 91177308-0d34-0410-b5e6-96231b3b80d8