multiplies by powers of two

[Matthew Fluet]

> As to the self compile: 2,829 is certainly a lot.  I wonder if it was loops
> counting down to 0 or more direct tests.

Probably some of each.  In just scanning though some other code, another
case I saw was comparing characters to #"\000".

> Did you end up putting in the tweak to notice that the flags were set by the
> last operation?  I doubt that it is a huge win, but ...

I just added it.  It hasn't triggered many on the few quick programs I ran
it through, but I'll see how it does on a self compile.  It's not
triggering any false-positives, so it shouldn't hurt performance to leave
it in.