nir: Add a structured flag to nir_shader

v2 (Jason Ekstrand):
 - Make "structured" a property of nir_function_impl not nir_shader
 - More validation and asserts

Signed-off-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/2401>
4 files changed