virtual stack regs.

[Paolo Bonzini]

> ARG_POINTER_REGNUM is included in the artificial defs of all blocks
> (which I think is overly conservative - just having them
> in the entry block def should be enough).
> Hence, from dataflow point of view, they are always considered initialized.
> 
> I think we should probably do something similar
> for VIRTUAL_STACK_*_REGNUM.

I think we should assert that we don't find virtual regs in dataflow, 
instead.

Paolo