radv: move optimizations in shader_compile_to_nir() to after io_to_scalar

This results in at least one less radv_optimize_nir() iteration.

No fossil-db changes.

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <mesa/mesa!6891>