v3d: Use nir_lower_io_to_scalar_early to DCE unused VS input components.

This lets us trim unused trailing components in the vertex attributes,
reducing the size of our VPM allocations.

src/broadcom/compiler/vir.c

@@ -585,6 +585,12 @@ vir_compile_init(const struct v3d_compiler *compiler,
         return c;
 }
 
+static int
+type_size_vec4(const struct glsl_type *type)
+{
+        return glsl_count_attribute_slots(type, false);
+}
+
 static void
 v3d_lower_nir(struct v3d_compile *c)
 {
@@ -713,6 +719,16 @@ uint64_t *v3d_compile_vs(const struct v3d_compiler *compiler,
 
         c->vs_key = key;
 
+        /* Split our input vars and dead code eliminate the unused
+         * components.
+         */
+        NIR_PASS_V(c->s, nir_lower_io_to_scalar_early, nir_var_shader_in);
+        v3d_optimize_nir(c->s);
+        NIR_PASS_V(c->s, nir_remove_dead_variables, nir_var_shader_in);
+        NIR_PASS_V(c->s, nir_lower_io, nir_var_shader_in,
+                   type_size_vec4,
+                   (nir_lower_io_options)0);
+
         v3d_lower_nir(c);
 
         if (key->clamp_color)

src/gallium/drivers/v3d/v3d_program.c

@@ -210,7 +210,10 @@ v3d_shader_state_create(struct pipe_context *pctx,
                 so->was_tgsi = true;
         }
 
-        NIR_PASS_V(s, nir_lower_io, nir_var_all & ~nir_var_uniform,
+        nir_variable_mode lower_mode = nir_var_all & ~nir_var_uniform;
+        if (s->info.stage == MESA_SHADER_VERTEX)
+                lower_mode &= ~nir_var_shader_in;
+        NIR_PASS_V(s, nir_lower_io, lower_mode,
                    type_size,
                    (nir_lower_io_options)0);