zink: do not read is_generated unless in tcs shader

It's undefined behavior in C to read a union member if another member
has been written to more recently. Let's be more careful here!

Fixes: a9d2b86c2c ("zink: store the spirv_shader to the zink_shader struct for generated tcs")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19457>

src/gallium/drivers/zink/zink_compiler.c

@@ -2348,7 +2348,7 @@ zink_shader_compile(struct zink_screen *screen, struct zink_shader *zs, nir_shad
    ralloc_free(nir);
 
    /* TODO: determine if there's any reason to cache spirv output? */
-   if (zs->is_generated)
+   if (zs->nir->info.stage == MESA_SHADER_TESS_CTRL && zs->is_generated)
       zs->spirv = spirv;
    else
       ralloc_free(spirv);