Even a BUG_ON() makes a measurable difference.  So remove some gratuitous
ones which will just trigger a null pointer deref anyway.

Also remove some debug code which isn't really being maintained any more.

Also replace (effectively):

	test_bit(N, foo);
	set_bit(N, foo);

with

	set_bit(N, foo);
	test_bit(N, foo);

In the first case we'll go onto the bus twice: once for the cache miss and
once to get exclusive write access.  In the second case we only go on the bus
once.  I think.  Certainly this trick chaved 40% off the cost of
shrink_list() when I did it there...

This patch is worth 1% or so on the bash script testing.