nir: Validate jump instructions as an instruction type

nir: Validate jump instructions as an instruction type

This has the downside of putting block successor validation in two
places that are a bit further apart.  However, handling them as a
special case makes the code more confusing than needed.  At least two
different people have not noticed that we don't have jump instruction
validation in the last week or two and added it.  Being able to search
for validate_jump_instr is useful.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5101>