nir: Turn imov/fmov of undef into undef

nir: Turn imov/fmov of undef into undef

Reverting the previous attempt at this a5502a721fd30fd resulted in
the following Vulkan test failing.

dEQP-VK.glsl.return.return_in_dynamic_loop_dynamic_vertex

This time we use the num_components from the alu dest rather than
num_inputs to the op to determine the size of the undef.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Cc: "13.0" <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99100