asahi: Fix type confusion for fragment shader keys

We can't attempt to access the fs union member if this is not a FS.
That worked so far since there wasn't a VS shader key at all, but we're
about to introduce one.

Signed-off-by: Asahi Lina <lina@asahilina.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23998>

src/asahi/compiler/agx_compile.c

@@ -2455,9 +2455,7 @@ agx_compile_shader_nir(nir_shader *nir, struct agx_shader_key *key,
     * Note: when epilogs are used, special handling is required since the sample
     * count is dynamic when the main fragment shader is compiled.
     */
-   if (key->fs.nr_samples) {
-      assert(nir->info.stage == MESA_SHADER_FRAGMENT);
-
+   if (nir->info.stage == MESA_SHADER_FRAGMENT && key->fs.nr_samples) {
       if (agx_nir_lower_sample_mask(nir, key->fs.nr_samples)) {
          /* Clean up ixor(bcsel) patterns created from sample mask lowering.
           * If this succeeds, we'll have expressions to constant fold to get the