In many cases loops have only one exit or a variable is only live
across one of the exits. In this case we know that all uses
outside of the loop will be dominated by the single LC PHI node
we insert. If that holds for all variables requiring LC SSA PHIs
then we can simplify the update_ssa proces
The following avoids collecting all loops exit blocks into bitmaps
and computing the union of those up the loop tree possibly repeatedly.
Instead we make sure to do this only once for each loop with a
definition possibly requiring a LC phi node plus make sure to
leverage recorded exits to avoid the
