radv: always use nir_lower_io_lower_64bit_to_32

radv: always use nir_lower_io_lower_64bit_to_32

Our I/O lowering doesn't handle 64-bit TCS stores and TES loads which use
several slots. Because of the large stride between slots, we have to split
the load so that there's a single load_buffer_amd/store_buffer_amd
intrinsic for each slot.

Our I/O lowering also sometimes creates nir_op_pack_64_2x32 after
nir_lower_alu_to_scalar.

Fixes KHR-GL45.gpu_shader_fp64.fp64.varyings with Zink
(https://gitlab.freedesktop.org/mesa/mesa/-/issues/6276)

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15863>