nir_lower_bit_size: Support lowering ops with differing source/dest sizes

Specifically the bit-finding routines always return int32. Don't complain
about the dest already being 32 bits when lowering to 32 bits, and
don't bother casting the dest if it's already right.

Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6313>