nir: Add optimization for fdot(x, 0) -> 0.

We had all these nice fdot opts to drop individual channels that were 0,
but nothing handling it being entirely 0!  Avoids r300g regression when
dropping them from GLSL.

Acked-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21475>

src/compiler/nir/nir_opt_algebraic.py

@@ -401,6 +401,10 @@ optimizations.extend([
 
    (('fdph', a, b), ('fdot4', ('vec4', 'a.x', 'a.y', 'a.z', 1.0), b), 'options->lower_fdph'),
 
+   (('fdot4', a, 0.0), 0.0),
+   (('fdot3', a, 0.0), 0.0),
+   (('fdot2', a, 0.0), 0.0),
+
    (('fdot4', ('vec4', a, b,   c,   1.0), d), ('fdph',  ('vec3', a, b, c), d), '!options->lower_fdph'),
    (('fdot4', ('vec4', a, 0.0, 0.0, 0.0), b), ('fmul', a, b)),
    (('fdot4', ('vec4', a, b,   0.0, 0.0), c), ('fdot2', ('vec2', a, b), c)),