agx: Call nir_lower_discard_if

We still need to implement discard itself, but this means we don't need
to worry about discard_if. This compiles down to the same idiom as
the vendor compiler (Metal) generates

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14217>

src/asahi/compiler/agx_compile.c

@@ -1481,6 +1481,9 @@ agx_compile_shader_nir(nir_shader *nir,
 
    agx_optimize_nir(nir);
 
+   /* Implement conditional discard with real control flow like Metal */
+   NIR_PASS_V(nir, nir_lower_discard_if);
+
    /* Must be last since NIR passes can remap driver_location freely */
    if (ctx->stage == MESA_SHADER_VERTEX) {
       agx_remap_varyings_vs(nir, &out->varyings, ctx->varyings);