More PRE compile-time optimizations

More PRE compile-time optimizations

This fixes a bug in bitmap_list_view which could end up with
a NULL head->current which makes followup searches fail.  Oops.

It also further optimizes the PRE DFS walk by removing useless
stuff and special-casing bitmaps with just one element for
EXECUTE_IF_AND_IN_BITMAP which makes a quite big difference.

2020-11-12  Richard Biener  <rguenther@suse.de>

	* bitmap.c (bitmap_list_view): Restore head->current.
	* tree-ssa-pre.c (pre_expr_DFS): Elide expr_visited bitmap.
	Special-case value expression bitmaps with one element.
	(bitmap_find_leader): Likewise.
	(sorted_array_from_bitmap_set): Elide expr_visited bitmap.