nir/inline_functions: Handle halting functions.

nir/inline_functions: Handle halting functions.

Without this stitch_blocks complains about ending in a jump with a
non-empty block after the inserted body.

I hit this with CTS raytracing tests where we tried to inline a
function that basically ended up being something like

{
   ignore_ray_intersection
   halt
}

I kept the nop path when possible as that does not leave a mess
for the optimization loop to optimize.

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12163>