nir/lower_bit_size: Fix subgroup lowering for floats

nir/lower_bit_size: Fix subgroup lowering for floats

Using u2u is always correct for integers, including signed integers,
because we're doing a down-cast.  It's wrong for floats, though.

Fixes: f95665cfebbd ("nir/lower_bit_size: Add support for lowering subgroup ops")
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25894>
(cherry picked from commit 5979e74177eb3013ca7e78867295e9a015e6c8ed)