Fix possible sanopt compile-time hog

Fix possible sanopt compile-time hog

While working on bitmap operations I figured sanopt.cc uses
a sbitmap worklist, iterating using bitmap_first_set_bit on it.
That's quadratic since bitmap_first_set_bit for sbitmap is O(n).

The fix is to use regular bitmaps for the worklist and the bitmap
feeding it and to avoid a useless copy.

	* sanopt.cc (sanitize_asan_mark_unpoison): Use bitmap
	for with_poison and alias worklist to it.
	(sanitize_asan_mark_poison): Likewise.