* Improve comments/documentation substantially

* Eliminate the releaseMemory method, this is not an analysis
* Change the fixed, active, and inactive lists of intervals to maintain an
  iterator for the current position in the interval.  This allows us to do
  constant time increments of the iterator instead of having to do a binary
  search to find our liverange in our liveinterval all of the time, which
  substantially speeds up cases where LiveIntervals have many LiveRanges
  - which is very common for physical registers.  On targets with many
  physregs, this can make a noticable difference.

  With a release build of LLC for PPC, this halves the time in
  processInactiveIntervals and processActiveIntervals, from 1.5s to .75s.

  This also lays the ground for more to come.

llvm-svn: 17933