tu: Call nir_lower_sysvals_to_varyings once

Remove the early call to nir_lower_sysvals_to_varyings. This guarantees
that we only call it after lower_system_values has replaced the
variables with intrinsics. It turns out that a sequence like this
doesn't work:

1. shader declares Layer builtin as a signed integer
2. nir_lower_sysvals_to_varyings turns it into a varying (but keeps the
  signed integer type)
3. nir_lower_input_attachments (or some other pass) creates a
  nir_load_layer_id intrinsic.
4. nir_lower_sysvals_to_varyings is called again, and when creating the
  varying variable it passes an unsigned type to
  nir_get_variable_with_location(), which asserts because there is
  already a signed integer variable.

By making lower_sysvals_to_varyings happen late for layer, we can avoid
this happening by lowering away the variable before (2).

Fixes: 5bbbf5cf9b ("tu: Set use_layer_id_sysval for nir_lower_input_attachments")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38793>

src/freedreno/vulkan/tu_shader.cc

@@ -224,12 +224,6 @@ tu_spirv_to_nir(struct tu_device *dev,
       nir_print_shader(nir, stderr);
    }
 
-   const struct nir_lower_sysvals_to_varyings_options sysvals_to_varyings = {
-      .point_coord = true,
-      .layer_id = true,
-   };
-   NIR_PASS(_, nir, nir_lower_sysvals_to_varyings, &sysvals_to_varyings);
-
    NIR_PASS(_, nir, nir_lower_global_vars_to_local);
 
    /* Older glslang missing bf6efd0316d8 ("SPV: Fix #2293: keep relaxed
@@ -2791,6 +2785,7 @@ tu_shader_create(struct tu_device *dev,
       NIR_PASS(_, nir, nir_lower_input_attachments, &att_options);
 
       const nir_lower_sysvals_to_varyings_options sysval_options = {
+         .point_coord = true,
          .layer_id = true,
          .view_index = true,
       };