i965/vec4: Make the vec4_visitor::nir_emit_instr default case unreachable

i965/vec4: Make the vec4_visitor::nir_emit_instr default case unreachable

The bug fixed by the previous commit went undetected because extra
stderr messages are not flagged by the CI.  Copy the solution from
fs_visitor::nir_emit_instr and mark the default case unreachable.

An alternate solution is to delete the default case so that the compiler
will issue a warning.  That may require more work since there are other
(impossible) cases that exist.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>