pan/bi: Handle vector moves

pan/bi: Handle vector moves

And fix the bad assertion that let this slip.

Like combines, nir_op_vec can be vector, and we need to lower this
ourselves. Thankfully, the lowering is simple.

Fixes
dEQP-GLES2.functional.shaders.loops.for_uniform_iterations.nested_tricky_dataflow_1_*

Fixes: b2c6cf2b6db ("pan/bi: Eliminate writemasks in the IR")
Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7081>