sbitmap cleanups

[John Marshall]

Richard Kenner writes:
> Right.  What does that have to do with "(x | *bp++) != *bp"?  That expression
> has its stored value modified exactly once!

The sentence right after the one Andrew quoted is "[...] the prior value
shall be accessed only to determine the value to be stored".  See also
http://www.eskimo.com/~scs/C-faq/q3.8.html

Thanks for updating the patch.

    John