Speed up LC SSA rewrite

Speed up LC SSA rewrite

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 intermediate bitmap allocation.

	* tree-ssa-loop-manip.cc (compute_live_loop_exits): Take
	the def loop exit block bitmap as argument instead of
	re-computing it here.
	(add_exit_phis_var): Adjust.
	(loop_name_cmp): New function.
	(add_exit_phis): Sort variables to insert LC PHI nodes
	after definition loop, for each definition loop compute
	the exit block bitmap once.
	(get_loops_exit): Remove.
	(rewrite_into_loop_closed_ssa_1): Do not pre-record
	all loop exit blocks into bitmaps.  Record loop exits
	if required.