nir: Assert that we don't shrink bit-sizes in nir_lower_bit_size()

nir: Assert that we don't shrink bit-sizes in nir_lower_bit_size()

The idea of this pass is to promote small bit-sizes to larger, supported
bit-sizes for certain operations.  It doesn't handle emulating large
bit-size operations on smaller bit-sizes; passes like nir_lower_int64
and nir_lower_doubles handle that.

So, assert that we aren't shrinking the bit-size, as this will almost
certainly produce incorrect results.

Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23123>