Fix the issues originally addressed in r54070.  After thinking about it some more, I realized that the right thing to do
is to have StrongPHIElimination use its knowledge of the PHIs before they're erased to update the intervals appropriate.  This is
both simpler and more accurate than the alternative, which was having LIA figure it out when it renumbered things, plus it's just
the right thing to do!

llvm-svn: 54077