Add some support for iterative coalescers to calculate a joined live

Add some support for iterative coalescers to calculate a joined live
range's weight properly.  This is turned off right now in the sense that
you'll get an assert if you get into a situation that can only be caused
by an iterative coalescer.  All other code paths operate exactly as
before so there is no functional change with this patch.  The asserts
should be disabled if/when an iterative coalescer gets added to trunk.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@76680 91177308-0d34-0410-b5e6-96231b3b80d8