pseudo-register addressing

[Stephen Weeks]

> If  the  lifetime  is ok, then couldn't you store the pseudo-registers on the
> stack?  The point being that this would save one register.

It would make sense to do this by patching the backend (not codegen) register
allocator to keep everything on the stack.  That should be easy to do, although
it would be a bit more work to try to shrink stack frame size.

With the x86-codegen now tracking stack slots, I can't remember if there is any
advantage to using pseudo-regs.