aco: Use aco_shader_info::hw_stage instead of guessing.

aco: Use aco_shader_info::hw_stage instead of guessing.

With this change, ACO is going to rely on the caller to set
the HW stage and will no longer guess it from the input shaders.

This will help enable compiling merged shaders separately,
but that will need further changes in instruction selection.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Qiang Yu <yuq825@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23597>