v3d: Don't rely on sorting input vars for VPM read setup.

v3d: Don't rely on sorting input vars for VPM read setup.

For supporting scalar VPM i/o at the NIR level, we need to do a pass over
the vars to figure out how big each attribute is after DCE.  Once we've
done that, we can just walk over c->vattr_sizes[] instead of bothering
with vars.