floor_log2_wide speedup

[DJ Delorie]

> > > If you want to micro-optimize this,
> > 
> > Not at the moment, no ;-)
> 
> Clearly you do, otherwise you wouldn't be changing anything at all.

Macro-optimize, yes.  Micro-optimize, no.  I'm not a big fan of code
that needs a ton of compiler-specific #ifdefs to do its job right.

Let's just say I still appreciate the simple solutions ;-)

I'm not against adding a comment and leaving it to the next
entrepreneur, though.