reload vs adds with clobbers

[DJ Delorie]

> > I'm currently testing adding an !N constraint after the O,P ones, to
> > catch the zero case, with an output template of ";", assuming that gcc
> > will always choose the P constraint for non-zero cases.
> 
> That should work, but it would be better to define a new constraint 
> letter to match only zero.  And of course you need to document why this 
> wierdness is there in the pattern.

Unfortunately, all the const_int constraints are used up in that port.
Does GCC play nice when you use an arbitrary constraint letter for
const_ints?