amd: lower subdword UBO loads in NIR

This fixes broken subdword UBO loads with LLVM.

It's only needed for LLVM, but it's done for both LLVM and ACO because
the pass can be fully validated only with ACO and the Vulkan CTS right now.

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19399>

src/amd/vulkan/radv_pipeline.c

@@ -3182,6 +3182,12 @@ radv_postprocess_nir(struct radv_pipeline *pipeline,
       }
    }
 
+   NIR_PASS(_, stage->nir, ac_nir_lower_subdword_loads,
+            (ac_nir_lower_subdword_options) {
+               .modes_1_comp = nir_var_mem_ubo,
+               .modes_N_comps = nir_var_mem_ubo
+            });
+
    progress = false;
    NIR_PASS(progress, stage->nir, nir_vk_lower_ycbcr_tex, ycbcr_conversion_lookup, pipeline_layout);
    /* Gather info in the case that nir_vk_lower_ycbcr_tex might have emitted resinfo instructions. */