virtual-stack-vars reference not resolved in vregs

[Paul Koning]

> On May 25, 2018, at 2:11 AM, Eric Botcazou <ebotcazou@adacore.com> wrote:
> 
>> Is this something the back end is responsible for getting right, for example
>> via the machine description file?  If so, any hints where to start?
> 
> The SUBREG of MEM is invalid at this stage.

Thanks.  That pointed me to the problem: the .md file contained a 
define_expand for truncsihi2, which is not useful given that the word
length is 2.  Deleting it cured the problem.

	paul