I was thinking "isn't this redundant with var->data_type->reg_size[...]?" and then discovered that this is the number of actually used registers, or at least it's supposed to be. That could use documentation :-)
Okay, I added the documentation.
I find a bit puzzling that the `count` field is not always initialized. I think we should either always initialize it, or document in which cases it is expected to be initialized.
I agree, `count` is now being initialized for buffers, semantics, and temps now.