nir/opt_algebraic: add trichotomy identity

In https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35802 we will
significantly rework geometry shaders & transform feedback. In the new approach,
transform feedback is executed as part of the hardware vertex shader, meaning
the vertex shader needs to write out all the "copies" of the same value into
different parts of the XFB buffer. In the general case of a GS writing triangle
strips, we get 0-3 copies. This is good and lets us parallelize XFB better with
GS.

In the case of a VS alone with XFB, we insert a passthrough GS. In that case
special case, we can only get at most 1 copy, so if we can prove the length of
the output strip is 3 we can delete 2/3 of the shader.

Anyway, the only thing preventing NIR from doing that optimization is failing to
see through some conditionals, fixed by optimizing with the law of trichotomy.

We could add other variants of this pattern (signed vs unsigned, iand vs
ior/ixor) if we expect anything else to hit this other than my boutique use
case.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Marek Olšák <maraeo@gmail.com>
Reviewed-by: Mary Guillemard <mary.guillemard@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35802>

src/compiler/nir/nir_opt_algebraic.py

@@ -1074,6 +1074,10 @@ optimizations.extend([
    (('iand', ('uge(is_used_once)', a, b), ('uge', a, c)), ('uge', a, ('umax', b, c))),
    (('iand', ('uge(is_used_once)', a, c), ('uge', b, c)), ('uge', ('umin', a, b), c)),
 
+   # Law of trichotomy. This pattern is load-bearing on AGX for optimizing
+   # emulated transform feedback.
+   (('iand', ('uge', a, b), ('ult', a, b)), False),
+
    # A number of shaders contain a pattern like a.x < 0.0 || a.x > 1.0 || a.y
    # < 0.0, || a.y > 1.0 || ...  These patterns rearrange and replace in a
    # single step.  Doing just the replacement can lead to an infinite loop as