multiplies by powers of two

[Matthew Fluet]

> How about with 4 -- two shifts (or adds) with intervening jo's?
> But that will probably be just as much Icache pressure as a mask and test.

I withdraw this question, because the peephole optimizer really isn't set
up for transforming a block into multiple return blocks, as would need to
be the case when inserting an additional overflow check.  (Maybe it would
work just to leave the extra jo in the block statements, but I'm not 100%
sure.)