nv50/ir: always return 0 when trying to read thread id along unit dim

nv50/ir: always return 0 when trying to read thread id along unit dim

Many many many compute shaders only define a 1- or 2-dimensional block,
but then continue to use system values that take the full 3d into
account (like gl_LocalInvocationIndex, etc). So for the special case
that a dimension is exactly 1, we know that the thread id along that
axis will always be 0, so return it as such and allow constant folding
to fix things up.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Pierre Moreau <pierre.morrow@free.fr>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>