Fix Dx12 naga codegen workaround

crates/bevy_pbr/src/prepass/prepass.wgsl

@@ -8,7 +8,6 @@
 // Most of these attributes are not used in the default prepass fragment shader, but they are still needed so we can
 // pass them to custom prepass shaders like pbr_prepass.wgsl.
 struct Vertex {
-    @builtin(instance_index) instance_index: u32,
     @location(0) position: vec3<f32>,
 
 #ifdef VERTEX_UVS
@@ -93,13 +92,14 @@ fn morph_previous_position(vertex: Vertex) -> vec3<f32> {
 #endif
 
 @vertex
-fn vertex(vertex_no_morph: Vertex) -> VertexOutput {
+fn vertex(
+    vertex_no_morph: Vertex,
+    // Separate the instance_index from the `Vertex` struct to avoid
+    // hitting https://github.com/gfx-rs/naga/issues/2416
+    @builtin(instance_index) vertex_instance: u32,
+) -> VertexOutput {
     var out: VertexOutput;
 
-    // Use vertex_no_morph.instance_index instead of vertex.instance_index to work around a wgpu dx12 bug.
-    // See https://github.com/gfx-rs/naga/issues/2416
-    let vertex_instance = vertex_no_morph.instance_index;
-
 #ifdef MORPH_TARGETS
     var vertex = morph_vertex(vertex_no_morph);
 #else