agx: fix uniform packing with local_load

fixes with constant promotion:

dEQP-VK.compute.shader_object_spirv.workgroup_memory_explicit_layout.alias.i32_to_u16_array_scalar_func_read_write_barrier

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31908>

src/asahi/compiler/agx_pack.c

@@ -234,7 +234,7 @@ agx_pack_local_base(const agx_instr *I, agx_index index, unsigned *flags)
       return 0;
    } else if (index.type == AGX_INDEX_UNIFORM) {
       *flags = 1 | ((index.value >> 8) << 1);
-      return index.value & BITFIELD_MASK(7);
+      return index.value & BITFIELD_MASK(8);
    } else {
       assert_register_is_aligned(I, index);
       *flags = 0;