nvc0/ir: avoid generating illegal instructions for compute constbuf loads

nvc0/ir: avoid generating illegal instructions for compute constbuf loads

For user-supplied constbufs, fileIndex is 0. In that case, when we
subtract 1, we'll end up loading from constbuf offset -16. This is
illegal, and there are asserts to avoid it. Normally we'd just DCE it,
but no point in generating the instructions if they're not going to be
used.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>