agx: Legalize LOD sources to be 16-bit only

I'm not convinced this is /right/ but it's a step.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11718>

src/asahi/compiler/agx_compile.c

@@ -1312,7 +1312,12 @@ agx_compile_shader_nir(nir_shader *nir,
       .lower_txp = ~0,
    };
 
+   nir_tex_src_type_constraints tex_constraints = {
+      [nir_tex_src_lod] = { true, 16 }
+   };
+
    NIR_PASS_V(nir, nir_lower_tex, &lower_tex_options);
+   NIR_PASS_V(nir, nir_legalize_16bit_sampler_srcs, tex_constraints);
 
    agx_optimize_nir(nir);