freedreno: Fix constant-index assumptions in IBO loads.

freedreno: Fix constant-index assumptions in IBO loads.

The encoder already sets up our IBO accesses as potentially nonuniform, so
we just need to be careful to not try to force the IBO index into an
immediate.

Fixes assertion failures in piglit arb_shader_image_load_store-invalid
(intermittent due to
https://gitlab.freedesktop.org/mesa/piglit/-/merge_requests/597), which
had some interesting actual failures hidden behind it.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13601>