Fix PRE topological expression set sorting

This fixes sorted_array_from_bitmap_set to do a topological sort
as required by re-using what PHI-translation does, namely a DFS
walk with the help of bitmap_find_leader.  The proper result
is verified by extra checking in clean () (which would have tripped
before) and for the testcase I'm working at during the last
patches (PR97623) it is neutral in compile-time cost.

2020-11-11  Richard Biener  <rguenther@suse.de>

	* tree-ssa-pre.c (pre_expr_DFS): New function.
	(sorted_array_from_bitmap_set): Use it to properly
	topologically sort the expression set.
	(clean): Verify we've cleaned everything we should.