pan/bi: Remove bogus assert for pack_32_2x16

pan/bi: Remove bogus assert for pack_32_2x16

The following IR is valid NIR:

   vec1 16 ssa_0 = ...
   vec1 32 ssa_1 = pack_32_2x16 ssa_0.xx

In this case, pack_32_2x16 takes in a two component vector, but the source
itself ssa_0 has only a single component. This is fine due to the shuffle, but
will fail the assert. Remove the assert and all is well.

Fixes test_relational.shuffle_copy.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18656>