Fix O(region-size) unwind in VN

Fix O(region-size) unwind in VN

This fixes the currently O(region-size) unwinding of avail info
to be O(unwind-size) by tracking a linked-list stack of pushed
avails.  This reduces the compile-time spent in complete unrolling
for WRF.

2021-02-09  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/98863
	* tree-ssa-sccvn.h (vn_avail::next_undo): Add.
	* tree-ssa-sccvn.c (last_pushed_avail): New global.
	(rpo_elim::eliminate_push_avail): Chain pushed avails.
	(unwind_state::avail_top): Add.
	(do_unwind): Rewrite unwinding of avail entries.
	(do_rpo_vn): Initialize last_pushed_avail and
	avail_top of the undo state.