nir: Fix printing of individual instructions with io semantics.

The state->shader is missing when used outside of nir_print_shader, just
drop these details in that case.  We can fix nir_print_instr() to look up
the shader, but let's also make sure that an instr detached from a shader
(such as one you're constructing but haven't yet inserted) still works.

Fixes: 2b1ef5df4eac ("nir: print IO semantics (v2)")
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6496>