V4 RegAlloc: store, pass, and use life-time intervals by pointer.

By storing LifeTimeIntervals by pointer (instead of by value), other
data-structures can safely use pointers too. This removes a lot of
copies, especially in vectors that act as worklists.

Also change the order of the "unhandled" list of intervals to be sorted
in descending order. Not only is this more efficient, but it also
removes the need to reverse the results of the life-range calculation
(which produces the list in exactly this order).

This change speeds up register allocation by about 20%.

Change-Id: I6ea3dcd110f250d9ccc881753dc7392510a26d87
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>